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The Philadelphia Phillies have placed right-handed pitcher Taijuan Walker on the injured list with right index finger inflammation stemming from a blister he developed during his start on Friday night against the Arizona Diamondbacks, the team announced Sunday. In a corresponding move, the Phillies will bring up right-hander Michael Mercado from Triple-A Lehigh Valley.

"We're going to try to work through that," manager Rob Thomson told reporters after Friday's game. "I think it's probably been going on for awhile. He's not going to admit to anything, because he's a competitor. He doesn't want to tell his opponents he's got a problem with his finger."

Thompson added that he believed Walker's finger had impacted his ability to throw his splitter, the best pitch in his arsenal. It's worth noting that Walker's four-seam fastball velocity was clocked in at a season-low 90.5 mph on average. His sinker also recorded one of its lowest average velocities of the year, at 90.4 mph. 

Walker, 31, struggled through four innings against the Diamondbacks Friday. He surrendered four runs on five hits and three walks, with three of those five hits clearing the fences. Walker's performance left his seasonal statistics at a 5.60 ERA (72 ERA+) and a 2.05 strikeout-to-walk ratio in 10 starts.

Mercado, 25, will be making his big-league debut. In 14 Triple-A appearances (10 of them starts), he's accumulated a 1.71 ERA and a 1.91 strikeout-to-walk ratio. 

Walker's next start is slated for Wednesday against the Detroit Tigers. It's unclear if the Phillies intend for Mercado to take that turn in the rotation, or if they'll lean instead on Spencer Turnbull, who threw 49 pitches as part of Friday's contest in relief of Walker.

The Phillies enter Sunday with a 50-26 record on the year, tying them with the New York Yankees for the best winning percentage in all of Major League Baseball.