PHILADELPHIA — Mets starter Frankie Montas is scheduled to undergo major elbow surgery today, likely taking him out of action until 2027, sources told The Post.

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Montas is expected to require full-blown Tommy John UCL repair based on imaging taken, which typically requires at least 12-to-14 months of rehab for starting pitchers. The right-handed Montas, one of Mets’ big free-agent signings this winter, was in the midst of a rough year on the mound after missing the first half of the season with a lat strain.

Montas, 3-2 with a 6.28 ERA, had just been demoted to the bullpen when he felt soreness that was diagnosed as a UCL tear in his right elbow.

Montas holds a $17 million player option for 2026, which he will obviously pick up. His total deal was for $34 million over two years and will be recalled as one that obviously didn’t work out. He is considered extremely talented and he has succeeded elsewhere, but his brief Yankees tenure also ended with injury and disappointment.