The Phillies need to make some changes to their lineup this offseason; that much has been obvious for some time. Maybe that's importing Bryce Harper or Manny Machado or both, but no matter how it happens, this is a group that had baseball's seventh-worst OPS+, ninth-fewest runs scored and some of the weakest defense we've seen in years.
The easiest way to begin solving the defensive problem is relatively simple. They need to take left fielder Rhys Hoskins, who was probably the least effective outfield defender in the National League last year, and move him back to first base, which is where he was playing before Philadelphia signed Carlos Santana last offseason. In order to do that, they'd likely need to trade Santana, and that's exactly what they may do, as they are reportedly shopping him heavily to other teams.
Forget the optics of potentially trading Santana after just one year of his three-year deal. If the right move comes along to improve the team on the field in 2019 and '20, then that's absolutely what the Phillies should do. If you're going to fix the defense, another year of Hoskins in left full-time is simply untenable. In order to figure out how to best achieve that, there are really three questions to answer, so let's dig into each one.
1. Do they have to trade Santana to move Hoskins back to first?
Probably! But not definitely.
2. What kind of player could interested teams expect to get in Santana?
Better than you might think, as we'll show you shortly.
3. Who might be interested in trading for Santana?
First base/DH types in their 30s aren't generally in high demand, but there is one thing about this offseason's market that does work in Philadelphia's favor.
Let's explain all that, in order.

1. The Phillies should probably trade Santana, but they don't necessarily have to.
If you want Hoskins back at first, the simplest thing to do is to trade Santana (and non-tender Justin Bour, probably). Then add Harper, or another outfielder like A.J. Pollock or Andrew McCutchen, and things look much cleaner.
But it's not a must-do, either. Third baseman Maikel Franco is a good bet to be traded, according to one report, and Santana did get into 19 late-season games at third. You wouldn't want him there on a daily basis if you're trying to fix the defense, but there's at least an argument for a job share where Hoskins only has to play left field once or twice a week, and Santana rotates between first, third and pinch-hitting. That's maybe not what you'd expect for the $35 million still due Santana, but it also doesn't matter: He's a quality player, and the Phillies can afford it. They need more quality players, not fewer.
Even in what was something of a down year for Santana -- more on that in a minute -- he was tied, essentially, with Hoskins and Cesar Hernandez for the best on-base percentage on the team. The Phils didn't have enough bats as it was last year; take him away, and now you need even more.
2. Santana's year was both "poor for him" and "better than you'd think."
Let's start with the place most people start. Santana had a .229 batting average. It was the lowest of any season of his career. That's enough for many to think that his first year in Philadelphia was a failure, but it's not that simple, mostly because teams don't use just batting average to evaluate hitters. (Not that it's not telling you anything here; while hitting lefty, Santana was shifted against 85 percent of the time, third most of any regular lefty hitter.)
That said, it really wasn't a great year for him. Santana's .352 on-base percentage, while still a strongly above-average figure (the 2018 non-pitcher Major League average was .323), was down from his pre-Phillies average of .365. His .414 slugging was his second-worst, and below his pre-Phils average of .445.
But it's important to remember just how rough Santana's April was. He hit only .153/.295 /.276 in his first month with the Phillies. It was the second-worst month of Santana's career; it was so rough that it was barely two weeks into the season when we called him "baseball's early hard-luck hitter," pointing out that he was still hitting the ball with such authority that his numbers would have to pick up.
They did, to an astonishingly consistent extent. Check out what Santana did from May 1 as compared to his career numbers or his 2019 projections. You can't tell them apart. They're identical.
After May 1:
.245/.364/.444, 119 wRC+

Career:
.247/.363/.442 ,121 wRC+

2019 Steamer projection:
.242/.359/.445, 119 wRC+
The point there is that Santana's lousy April shouldn't meaningfully change what you think about him. In addition, by walking 110 times against only 93 strikeouts, Santana gained entry into a pretty exclusive club. Only three other players took at least 400 plate appearances and walked as much as (or more than) they struck out. The other three?Joey Votto, Alex Bregman and Jose Ramirez. That's a list you want to be on.
Think about it this way: Santana may not be a fit on the 2019 Phils because of the way their roster is constructed, but that doesn't mean he's not still a quality big league hitter. It just might need to be somewhere else.

3. The main reason teams might be interested in trading for Santana -- and who might do it.
Take a look at the current list of free agents, focusing on first basemen. What you'll notice there is that there is not one starting-caliber first-base option available. World Series hero Steve Pearce is probably the most notable name, but he'll be 36 and has never taken even 400 plate appearances in a season. 
That means if you want a first baseman, you're trading for Paul Goldschmidt, who is a superior player to Santana but would cost far more, or perhaps Jose Abreu, if the White Sox are willing to let him go. Santana is projected to be the 12th-best first baseman in 2019; while that doesn't include Hoskins, it also sounds about right, because it's slightly better than average.
So, let's assume that the Phillies would take on some of his remaining salary. Where could we find fits?
Rockies
Last year's Colorado team was fueled by outstanding starting pitching, but ultimately fell short due to a lack of offensive depth. Nowhere was that more noticeable than at first base, where the Rockies had a .232/.314/.405 line that was the third-weakest in baseball. Headed into 2019, they're projected in the same range, tied for fourth-weakest, because Ian Desmond's .236/.307/.422 (81 wRC+) last year just wasn't good enough; he ought to be moved into a multi-positional backup role.
The Rockies could just go with ready-now Ryan McMahon, but he hasn't proven much yet, and there's plenty of ways to find time between the lefty McMahon, who can also play second or third, and the switch-hitting Santana. A contact hitter like Santana would also be an interesting fit in Denver, as the enormous Coors Field outfield could help him find a few extra hits. 
Twins
Ideally, however, Santana goes to an American League team where he can take some time as a DH. Minnesota fits the bill well, because now that Joe Mauer is retired and Logan Morrison is a free agent, the Twins have plenty of time available at first base and DH, even if Christopher Austin is likely to get some opportunities there. If you combine those two spots, Minnesota had the third-weakest 1B/DH combo in 2018, so even a slightly above-average hitter like Santana would be an upgrade, as well as providing insurance for the relatively unproven Austin.

Mariners
It's a little difficult to know how Seattle is approaching 2019, though we do know there was reportedly some interest in Santana last year. We also know that general manager Jerry Dipoto is a more frantic trader than anyone else in the game, and you can expect more of that this offseason.
"We've not been huge players in free agency to begin with," Dipoto said last week. "A lot of that will be defined by what we wind up doing by trade. I'd never say never, but I'd say that's not our first path."
Last year's DH, Nelson Cruz, is a free agent, and Ryon Healy (who hit .235/.277/.412 in 2018) is no roadblock. The Mariners are projected for the 25th-best first-base performance in 2019, though it remains to be seen how much time Robinson Cano gets there.
Angels
Any year with Michael Trout is a year you need to try to win in, and the Angels may only have two of them left. While they need starting pitching help, they could also use a bat, too, and their first-base situation is dire, projected to be the second weakest in baseball. That, of course, has everything to do with the fact that Shohei Ohtani should get the bulk of the plate appearances at DH, forcing Jose Pujols once again to first base. The Halos would be better off bidding Pujols farewell and trading for Santana, though we admit it's unlikely. 
Santana is a solid bat, last year's slow start aside. The market is rarely friendly to over-30 players who are 1B/DH types, and perhaps this year won't be any different, so maybe staying in Philadelphia is the best outcome -- there's no value in trading him just to trade him. If not, however, someone will be interested. There's a fit out there.