General manager Sandy Alderson said Santana reported the arm issue on Monday after a side throwing session. Alderson described the delay as more of a schedule tweak than a setback.
"He came back, didn't feel quite as strong as he would've liked. So rather than force it (we're) backing off a day," he said on Monday. "We'll see how he feels tomorrow. But again, we're not, I wouldn't view that as a setback necessarily; I think it's a tweak in the schedule. We'll know tomorrow whether it constitutes something more significant."
Santana underwent surgery to repair a torn anterior capsule in his left shoulder last September. He has missed 108 games (and counting) due to the injury.
The Mets are still hoping that Santana will return at some point this season.
Alderson said there's no reason to rush Santana's recovery, and the team will know Tuesday if there is any further change to the pitcher's schedule. If he has no major setbacks, the two-time Cy Young Award winner could return by the end of August.
Last Thursday, in his first rehab start, Santana allowed two hits and plunked a batter in three scoreless innings while striking out three against the Daytona Cubs. He required only 33 pitches for the three innings, under the cap of 45. Reports had Santana at 89-90 mph, with a dip in his final frame.
Ian Begley is a regular contributor to ESPNNewYork.com. Information from The Associated Press was used in this report.