Alderson said deGrom threw an effective bullpen session on Friday "and felt great." But when deGrom made a throw in the outfield while shagging fly balls during batting practice after that, he experienced some pain. That's when the Mets decided to shut him down.
deGrom underwent Tommy John surgery on that elbow in October 2010. But Alderson said the pending surgery wouldn't be as extensive.
"Jacob has had issues with the ulnar nerve in his right elbow, which is not unusual after Tommy John surgery, even during the time after that surgery," Alderson said. "He will not pitch tomorrow. I think it's unlikely he will pitch the rest of the season. We'll see.
"It's likely that this will require a surgical repair. It's not a significant surgical procedure at least with respect to risk, moving forward. But it's something that will have to take place at some point."
deGrom is one of three Mets starters who came up through the system to undergo Tommy John surgery.
Matt Harvey is already out until 2017 after losing feeling in his right arm and right fingers because of a compressed nerve. He had a rib removed in surgery to relieve what is called thoracic outlet syndrome, and he hasn't pitched since July 4.
Steven Matz is working through a sore left shoulder, and he is currently in Florida on a rehab assignment. Matz hasn't pitched since Aug. 14. There's no timetable right now for his return, Mets manager Terry Collins said on Saturday.
Zack Wheeler, another Mets starter, had Tommy John surgery last year. Wheeler, obtained in the 2011 deal that sent Carlos Beltran to the Giants, was shut down during his rehab this summer and there's also no timetable for his return.
deGrom had allowed 16 runs on 31 hits with seven walks in his final three starts, covering 14 2/3 innings for a 7.98 ERA during that span. For the season, he's 7-8 with a 3.04 ERA and 143 strikeouts in 24 starts, covering 148 innings.
In his last start against the Marlins on Sept. 1, deGrom pitched five innings. He allowed three runs on six hits, walked four, struck out six and threw 102 pitches. He looked like he was struggling with his command and control, uncharacteristically falling behind most Miami hitters.
When deGrom walked off the mound that night, he signaled trainers, and unbeknownst to his manager at the time, told them he was having arm issues. A subsequent MRI revealed no structural damage, but deGrom was put on anti-inflammatory medication and rested for five days before he resumed throwing.
"He was hoping to pitch through it," Alderson said. "But this thing flares up at unpredictable times and under unpredictable circumstances. So I think it's unlikely he'll pitch again this season."