PHILADELPHIA -- Spencer Turnbull's return to the Phillies' starting rotation was short-lived.

The right-hander was placed on the IL with a right lat strain on Thursday, one day after leaving Wednesday's outing in Detroit after just three innings.

"It's probably going to be six weeks, could be eight weeks," manager Rob Thomson said.

The Turnbull move comes just four days after No. 5 starter Taijuan Walker was placed on the 15-day IL due to right index finger inflammation. There remains no timetable for Walker's return, though it "won't be before the All-Star break," per Thomson.

That leaves Michael Mercado, the club's No. 30 prospect, as the next man up in the rotation. He will be available out of the bullpen for the next couple nights before slotting back into the rotation sometime next week. The No. 5 spot would tentatively come up on Tuesday against the Cubs in Chicago, but Monday's off-day could allow the Phillies to shuffle their rotation by one day if they choose.

"I came up here with the knowledge that I'd be in the bullpen at first, but you never knew what could happen," Mercado said. "I've done both, obviously, so either way, it's just fun to be up here."

Initially recalled on Sunday to fill Turnbull's spot in the bullpen, Mercado should now get an extended look in the rotation for the next month. He had a 1.71 ERA over 14 outings (10 starts) with Triple-A Lehigh Valley before tossing a 1-2-3 inning in relief in his MLB debut on Monday.

"He's got a great arm," Thomson said. "Power stuff. He spins the ball really well and he throws strikes. He's had a really good year at Triple-A and the one inning he threw in Detroit, he was really poised."

Mercado began the year in the bullpen for Lehigh Valley before director of pitching development Brian Kaplan suggested the club move the 25-year-old righty into the rotation to provide more organizational depth.

"He saw the velocity and the stuff and the strike-throwing ability and different pitches he can throw," Thomson said. "So just with [Mercado's] arsenal, [Kaplan] thought he'd be a good starting piece for us for some depth -- and he was right.

"It was a good idea and thank God he did it."

Mercado's first stint as a big league starter will come in a rotation featuring Zack Wheeler, Aaron Nola, Ranger Suárez and Cristopher Sánchez. Philadelphia's top four starters have formed the most dominant starting quartet in the Majors this season.

"It's incredible just to be part of that," Mercado said. "Hopefully, I'll be able to pick their minds about certain things. Obviously, they have a ton of experience and it's just so cool to be surrounded by those types of guys."

At this time a week ago, the Phillies seemingly had six starters for five spots with Turnbull relegated to relief duty. Now, they've suddenly dipped into their depth -- and it's unclear who would be the next man up, if needed.

Thomson mentioned Triple-A starters Tyler Phillips (5.02 ERA) and David Buchanan (5.19) as further potential depth options. Mick Abel, the club's No. 3 overall prospect, is 1-7 with a 7.08 ERA in 12 starts. He has a 9.69 ERA and 10 walks over 13 innings in three starts this month.

It's certainly an area the Phillies could address in the coming weeks, especially considering they haven't used fewer than 10 starters in any season since 2014. Mercado will be just their seventh starting pitcher this season -- with the only changes coming in the No. 5 spot.

"It's a long year," Thomson said. "It's tough -- 162 [games] is tough. So you've got to have depth and our front office has done a great job of getting us that. We've just got to keep moving forward."

The Phillies expect Turnbull to be back in enough time to stretch back out if he's needed as a starter down the stretch. They're also hoping Walker's blister heals to the point that he can begin effectively gripping his splitter again.

In the meantime, Mercado will take the ball every fifth game for a team that entered Thursday with the best record in the Majors.

"This is what you want to do," he said. "There is nothing more fun than when you're on a really good team that's winning. So no matter what happens, this is just a great opportunity and I'm doing whatever I can to make the most of it.