Yankees' Jordan Montgomery having Tommy John surgery

A new MRI taken Monday on Montgomery revealed a tear in the left-hander’s elbow.

TORONTO— The first month of Yankees left-hander Jordan Montgomery’s recovery from an elbow strain went well.

As of early last weekend, Montgomery was back playing catch and possibly only 2-to-4 weeks away from being ready to challenge replacement Domingo German for his old No. 5 starter spot.

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Then came a setback last Saturday in Baltimore when the second-year pitcher played catch. Montgomery didn’t feel right and stopped throwing, and that led to another MRI on Monday morning.

This one revealed a much worse diagnoses:

Montgomery has tear in his ulnar collateral ligament and needs Tommy John surgery, and a quick decision was made to go through with the procedure at New York-Presbyterian Hospital on Thursday.

Obviously, Montgomery’s season is over and he might not be back until late 2019 or even 2020, depending on how his long rehab goes.

“I’m obviously disappointed for Monty, but at least he gets some answers now and know he’ll be in really good hands,” Yankees manager Aaron Boone said Tuesday before his club played the Toronto Blue Jays at Rogers Centre. “Get through the surgery and start that rehab process.

“I talked him today and he’s in pretty good spirits considering, so we’ll just give him the best support we can and look forward to him to getting back at some point.”

Montgomery, 25, was 2-0 with a 3.62 ERA in six starts this season after going 9-7 with a 3.88 ERA in 29 starts as a rookie in 2017.

Elbow tightness forced Montgomery out of his last start after one inning May 1 in Houston, then he was put on the 10-day disabled list the following day with a injury that was termed an elbow strain. At the time, the Yankees put a 6-to-8 week timetable on Montgomery returning.

“We thought it was going to be the timeline that we were on,” Boone said. “I think there’s probably a little gray in there with the original diagnoses, but we were hoping that he was on his way to building back up to be able to pitch for us again this year. But obviously that’s not the case.”

With Montgomery done for the year, German now has a firm grip on the Yankees’ fifth starter spot. The righty is 0-3 with a 5.54 in 10 games, five as a starter, but he’s pitched better than his stats.

“Well, obviously Domingo remains where he is and he’s done a great job for us, so we feel really good about the five that we have right now,” Boone said. “But obviously we’re a little bit nicked up. We’ve got some guys that we feel like can contribute at some point if we got into a situation and obviously (GM) Brian (Cashman) and the whole organization are always working as many possibilities as we can to continue to keep the depth viable.”

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