Syndergaard (elbow) was removed from his rehab start with Low-A St. Lucie on Tuesday after one inning, Tim Healey of Newsday reports.
The 28-year-old went four innings during his first rehab outing last Wednesday, and he was expected to build on that Tuesday. According to Healey, Syndergaard saw his fastball velocity drop to the low 90s by the end of his lone inning, which could be a worrying development as he returns from Tommy John surgery.