The Phillies recalled right-hander Zach Eflin to take Knapp's spot on the 25-man roster. Eflin started Tuesday night against the Braves at SunTrust Park.
Cameron Rupp is likely to handle the bulk of playing time while Knapp is on the DL, but Alfaro will play. Alfaro is the No. 70 prospect in baseball, according to MLBPipeline, but he remains unpolished both offensively and defensively. He hit .241 with seven home runs, 43 RBIs, 16 walks, 113 strikeouts and a .649 OPS in 350 plate appearances with Triple-A Lehigh Valley, but just .214 with a .580 OPS since April.
Alfaro went 2-for-4 Saturday against the Rockies.
"I'll pick my spots, but I'll play him," Mackanin said about Alfaro. "I can't catch Rupp every day. He'll get a good bit of playing time."
Alfaro has a chance to open some eyes while Knapp is sidelined. Alfaro is out of options next year, which means he must be on the Opening Day roster or the Phillies risk losing him. If the Phillies like what they see -- or even if they don't, but continue to believe in his potential -- they could try to trade Rupp or Knapp in the offseason.
Todd Zolecki has covered the Phillies since 2003, and for MLB.com since 2009. Follow him on Twitter and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.