The Mets are not prepared to announce the decision until deGrom is examined in New York later Monday. DeGrom has a sore shoulder.
Terry Collins insisted Monday morning that Montero would not be activated if deGrom avoids the DL. Instead, the Mets would handle a spot start in-house.
Anthony Gruppuso/USA TODAY Sports Rafael Montero is expected to start Tuesday in place of Jacob deGrom.
Montero told Dominican journalist Hector Gomez in Spanish: "I learned very much in the little time I was in MLB. I hope to stay up the rest of my career. I have worked hard to be here. I'm coming off two very good starts. I feel like I'm in my best physical form of the whole year."
Montero, 23, went 0-2 with a 5.40 ERA in four major league starts in May. He arrived with a reputation as a strike-thrower, but nibbled and walked 11 in 20 innings.
Montero was selected over Noah Syndergaard as well as using Carlos Torres for a spot start.
Montero was due to pitch Monday for Triple-A Las Vegas, so the start for the Mets will come on an extra day of rest.
He has tossed 14 2/3 scoreless innings over his past two Pacific Coast League starts, allowing a combined four hits and four walks while striking out 17. Overall, he has made four starts since missing more than a month due to a strained left oblique.