Syndergaard will make his MLB debut by starting Tuesday's game against the Chicago Cubs.
Alderson said Gee, who was scheduled to start Sunday, has a mild groin strain.
Syndergaard was ranked as Baseball America's No. 11 prospect heading into 2015. The 22-year-old is 3–0 with a 1.82 ERA and 34 strikeouts in 29 2/3 innings at Triple A Las Vegas this season.
The Mets (18–10) are playing the Phillies in Philadelphia on Friday.
- Molly Geary