PHILADELPHIA -- Taijuan Walker will miss at least his next few starts as he continues his ongoing search to find his splitter.

The Phillies placed the veteran right-hander on the 15-day injured list prior to Sunday's series finale against the D-backs due to right index finger inflammation. Manager Rob Thomson said there is also a blister on the inside of the finger that has impacted Walker's ability to throw his most important pitch.

Philadelphia recalled Michael Mercado (the club's No. 30 prospect) from Triple-A Lehigh Valley. He will pitch out of the bullpen, while Spencer Turnbull slots back into Walker's rotation spot.

It's unclear at this point how long Walker will be sidelined.

"Don't know, don't know," Thomson said. "It all revolves around being able to throw a split. That's what's really affecting him."

The finger issue is specifically impacting Walker’s ability to grip the splitter. Thomson said Walker can still throw his fastball -- and thus, he will be able to continue throwing while on the IL -- but the seams rub against the blister when he grips the splitter.

Walker's velocity has been down this season -- his sinker is averaging 90.9 mph after averaging 92.4 mph in 2023 -- but the real issue is that splitter. As someone who throws in the low 90s, Walker can still be effective with diminished velocity … but only if the splitter is working.

And it hasn't been.

After a rough start to the 2022 season with the Mets, Walker tried out a new grip on his splitter -- and it instantly became one of the best pitches in the game. Opponents hit just .195 with a .267 slugging percentage and a 29% whiff rate against the pitch, which he used more and more as the season went on.

In his first year with the Phillies, the splitter became Walker's most-used pitch. He threw it 33.2% of the time and opponents hit just .205 with a .295 slugging percentage against it.

But this season, Walker has been unable to throw the splitter low in the zone. He's consistently left it up in the zone and the opposition has made him pay to the tune of a .426 average (23-for-54) and a .704 slugging percentage.

In that 2022 season, only one Major League pitcher (Tony Gonsolin) threw a more effective splitter than Walker, according to Statcast's run value metric. By that same metric, only one pitcher (Toronto's Erik Swanson) has thrown a less effective splitter this season.

Walker has all but abandoned the pitch in his past two outings, throwing just seven and nine, respectively. He hadn’t thrown fewer than 13 in any of his previous starts since joining the Phillies.

Though it remains to be seen how long Walker's IL stint lasts, the club is hoping it serves as a reset for a pitcher who has a 5.60 ERA in 10 starts in the second year of his four-year, $72 million deal.

"I hope it works that way, sure," Thomson said. "We can get back to doing some work with him -- delivery work and things like that -- until he gets to be able to throw his split."

In the meantime, Turnbull will return to the rotation starting on Wednesday in Detroit against his former team. The righty went 2-0 with a 1.67 ERA in six starts while Walker was on the IL with right shoulder soreness to begin the season.

Turnbull had his ups and downs after transitioning to the bullpen, but he's confident that getting back to his typical routine will help him pick up where he left off as a starter.

"I've enjoyed being in the bullpen and learning everything," Turnbull said. "Like I've said, I want to do whatever I can to help the team win, but obviously I prefer starting. It's what I've always done and feel more comfortable with."

Mercado, meanwhile, will assume Turnbull's role in the bullpen. The 25-year-old prospect posted a 1.71 ERA over 14 outings (10 starts) with Lehigh Valley this season.

"He's really talented," Thomson said. "He's had a good year. He's got a great arm. He's a really good kid. So yeah, I'm excited for him."