The Red Sox outfield in 2018 was -- and this should really go without saying -- just fantastic.

Mookie Betts had an all-time season on his way to winning the Most Valuable Player Award. Andrew Benintendi improved in almost every way from the 2017 that placed him second in the Rookie of the Year balloting. Jackie Bradley Jr. won his first Gold Glove and showed signs of offensive life down the stretch. And their "fourth outfielder," at least when he wasn't in the lineup as designated hitter? That was J.D. Martinez, who merely smashed 43 home runs and finished fourth in the MVP balloting himself.

They were great, clearly. Just looking at production during games played in the outfield, so not even counting Martinez' time at DH, the Red Sox outfielders were first in average (.300), on-base percentage (.380), slugging percentage (.518), OPS (.899), wOBA (.381), wRC+ (139), Wins Above Replacement (18.6), and ... well, you get the point. The Red Sox won 108 games and the World Series. The outfield was a big part of why.

They were so great, in fact, that they ranked as one of the best collections in history. That's fun enough, but let's also ask the far more interesting question: What do the 2019 Red Sox have to do to have the best outfield of all time?

It won't be easy. But it's not impossible, either. Here's what they're up against.

The 2018 Red Sox outfield was already historically remarkable

There's a tricky aspect to all of this, obviously. What's an outfielder, and when? Do we count all 614 of Betts' plate appearances, or just the 592 he had while playing outfield? If we do take all of his times to the plate, what do we do about Martinez, who had only 249 plate appearances in the outfield corners, and 400 more as a designated hitter? Different sites approach this differently, meaning you'll get different numbers to the same question.

For our part, we wanted to try to be as accurate as possible. At Baseball-Reference, they allow you to split a team's positional production by something closer to actual playing time, so the best-in-baseball -- by nearly double! -- 10.6 Wins Above Replacement the Red Sox compiled in right field, for example, isn't just Betts. It's also the limited time spent there by Martinez, Bradley, Blake Swihart, Brock Holt and each of the seven Boston players who spent time there.

It wasn't easy to go back a century. We did it anyway. Looking at it this way, the 2018 Red Sox outfield was ... the 15th-best in baseball history. Sort of.

The "sort of" is because there's no real meaningful difference in tenths of a point of WAR, so the 2018 Sox are essentially in a big tie around 19 WAR for something like ninth place. That's especially true because defensive metrics may not be perfect now and they're certainly less reliable as we go back in time, but because fielding is such a big part of what made the 2018 Sox outfielders so good, we can't look at hitting only, so that's why we're using Wins Above Replacement.

This list makes sense, clearly. Of course Babe Ruth's 1927 Yankees, arguably the most famous baseball team of all time, top the list. (Really, this list is full of various historic Yankee teams, from the Ruth era to the DiMaggio teams to the famous Maris/Mantle club of 1961.) Those 1980 A's had 21-year-old Rickey Henderson stealing 100 bases with a .420 on-base percentage and Tony Armas smashing 35 homers. Maybe you don't think about the 1962-63 Giants very much, but that team had Willie Mays, Willie McCovey and Felipe Alou at the height of their powers in the outfield.

Merely being in this conversation says a great deal about just how good the Red Sox outfielders just were. Now we have a target to hit, which is three to four more wins. What do the 2019 Red Sox need to do to get there?

What the 2019 Red Sox outfield can -- and will need to -- do better

1. Bradley's offensive breakout needs to be for real.

Bradley's been around Boston for so long that when he made his Red Sox debut back in 2013, he was playing left field next to Jacoby Ellsbury. Mike Napoli was the first baseman, and Jon Lester was pitching. Now entering his seventh season in Boston, his tenure so far has been full of ups and downs, with a dreadful year in 2014, solid ones in 2015 and 2016, and inconsistent ones in 2017 and 2018.

Maybe, after all this time, it's not realistic to expect a sustained new level of offensive production, but it does feel different this time. As we investigated in January, Bradley is one of 2019's most obvious breakout candidates. He's got elite, 96th percentile hard-hit rate, and he spent time this winter working with Craig Wallenbrock, the hitting coach who helped improve Martinez, Chris Taylor and others.

Remember this wild quote that he gave to WEEI in December?

"This is the first time I heard any of this stuff. What I've been taught my whole life is completely wrong. It's scary to say that, but it's wrong. I feel fortunate enough to make it this far doing it wrong."

Imagine how talented you must be to feel like you're "doing it wrong" and still be the center fielder on a World Series winning team. In the second half, Bradley hit a strong .269/.340/.487 (about as good as Freddie Freeman). For what little Spring Training is worth, he's slugging .591. We're in on this.

We're not going to go nuts and turn him into Aaron Judge or anything, but let's say he can take 2016, his previous best hitting season, and perform that way for a full season, when he was worth +5.5 WAR. His .267/.349/.486 from that year is almost exactly what he just hit in the second half, anyway. With his defense, it's not crazy.

• Bradley's 2018 outfield WAR: 2.1
• Bradley's best-case 2019 outfield WAR: 5.5
• Bradley's net change: +3.4

• Boston 2019 outfield WAR added: +3.4

So, that's a good start. Throw in a career year from Bradley into last year's outfield, and we're already there. Except...

2. Betts needs to perform at or near his 2018 level

... this is unfair. This isn't a reasonable expectation to have for anyone. Betts just put up a season that was very literally one of the greatest of all time, and if you think it's hard to do that once, think about how hard it is to do that multiple times. Only Ruth, Bonds, Mays and Mantle have put up multiple years like this. (This is why Mike Trout has far surpassed Bryce Harper in the "game's greatest player" conversation, because Harper has had one historic year, while Trout does it every single season.)

But we're trying to get the Red Sox to the best outfield of all time, right? And no one believes that Betts' 2018 was some sort of fluke, right? No, it's not fair at all to expect that Betts can put up another season like he just did, but it's not out of the question that he can. For our purposes, he'll have to do something like it. You don't top the 1927 Yankees without some historic performances.

Even so, let's be reasonable and downgrade him slightly. The reasonable and somewhat more conservative ZiPS and Steamer projections have him down for a seven-win season. We're trying to have fun here, so we'll still say nine. It'll still be a great season. Just a somewhat less-great season.

• Betts's 2018 outfield WAR: 10.5
• Betts's more reasonable but still absurd 2019 outfield WAR**: 9.0**
• Betts's net change: -1.5 WAR

• Boston 2019 outfield WAR added: +1.9

3. Martinez does, too.

This one's a little easier. We're now five years into Martinez's well-documented career rebirth as one of the most dominating sluggers in the game. It's true that his .330/.402/.629 in 2018 was impressive, but it's also true that it's not that much better than the .305/.374/.594 line he's put up dating back to 2015. Barring serious injury, there's little reason to expect anything different this year, or any additional time in the outfield.

• Martinez's 2018 outfield WAR: 2.5 (remember, we're not counting DH time)
• Martinez's 2019 outfield WAR**: 2.5**
• Martinez's net change: 0.0 WAR

• Boston 2019 outfield WAR added: +1.9

4. Benintendi has more or less the same season

Benintendi had a good 2018, not an elite one. There's nothing wrong with .290/.366/.465 to go with 16 homers and 21 steals, obviously. Plus, he may get more time atop the lineup this year, ahead of Betts. That should gain him a few more plate appearances over the course of the year. That's all good.

But for all of his obvious talent, it's difficult to realistically forecast a sudden extra breakout, in part because he was already good, and in part because a 31st-percentile hard-hit rate is below average. Put another way, similar names to him in terms of 2018 hard-hit rate were Neil Walker, Dansby Swanson, Adeiny Hechavarria and the injured versions of Carlos Correa and Kris Bryant.

He has above-average plate discipline, obviously, and he's not even 25 until July. It's possible there's more yet to come. But the Steamer and ZiPS projections each expect something like the exact same season he just had. It was a good one. We'll go with that too.

• Benintendi's 2018 outfield WAR: 3.9
• Benintendi's 2019 outfield WAR**: 3.9**
• Benintendi's net change: 0.0 WAR

• Boston 2019 outfield WAR added: +1.9

5. The other guys have to give you something, or at least not hurt you

Assuming health, there's not a lot of extra playing time available here. Last year, Swihart, Holt, Steve Pearce, Sam Travis and Tzu-Wei Lin combined for just 122 outfield plate appearances and 0.0 WAR. The depth chart hasn't changed much, so expect more of the same. They don't really have to add much, though it'd be nice. They just have to not be a negative. Seems reasonable.

• Backups 2018 outfield WAR: 0.0
• Backups 2019 outfield WAR: 0.0
• Backups net change: 0.0 WAR

• Total Boston 2019 outfield WAR added: +1.9

* * *

So: If that's what happens, that Bradley blows up, that Betts has another great season, that Benintendi, Martinez and the rest stay about the same, then we're looking at 20.5 WAR, or basically tied with the '63 Giants for the third-best outfield of all time. If Betts really does repeat all of his 2018 season, or if Benintendi takes a step forward, the '27 Yankees would be in reach.

It's not likely. It's not probable. Just about everything would have to go right, with no injuries. It's not so much that this is going to happen, because it probably won't. It's that you can even have this conversation with a straight face. The 2018 Red Sox outfield was so, so good. The 2019 version could be something historic.