To say that Rafael Montero, the prospect once valued higher than Jacob deGrom in the Mets system, hasn’t lived up to expectations would be an understatement.

The now 26-year-old has shown time and time again that his stuff is good, but if he could just get his control down pat and work through his mental hurdles on the mound, he could be a threat in the Mets rotation.

Monday night at Yankee Stadium, Montero showed glimpses of what the Mets hoped they were getting when he made his major league debut against the Yankees in May of 2014.

“I hope it’s a big step forward,” Mets manager Terry Collins said. “He has the stuff to pitch here. If he locates it and trusts his stuff and throws the ball in the strike zones, he has a chance to get outs and he showed that tonight.”

The right-hander went six strong innings in the Bronx, allowing just two runs and scattering five hits while collecting six strikeouts.

Montero squared off against former Met farmhand Luis Cessa, most-famously being half of the return the Detroit Tigers got in exchange for Yoenis Cespedes two years ago.

In the third inning, the Mets got on the scoreboard with solo home runs from Curtis Granderson and the aforementioned Cespedes.

The Yankees were able to scratch out their first run when Gary Sanchez cut the deficit in half for the Yankees with a sacrifice fly in the fourth inning, but the results could have been a lot worse. Montero worked out of a tough jam to mitigate the damage and prevented it from getting out of hand.

“The fourth inning could have been disaster for us and he hung in there and battled and got out of there with one run and I thought it was a big turnaround for him,” Collins said.

However, Aaron Judge crushed his 36th big fly of the year in the sixth to sour Montero’s outing and stick him with the tough luck no-decision.

“I threw the sinker a lot tonight and I felt like I had good results with that,” Montero said. “I think all my pitches were really working tonight. Thanks God. (Judge) got me, he had a really good hit on one of my pitches.”

The first man out of the bullpen for the Mets was Hansel Robles, who turned in a clean seventh inning but was tagged with a homer off the bat of Aaron Hicks to lead off the eighth.

Erik Goeddel served up another long ball later in the inning to Gary Sanchez, but the Mets couldn’t catch up as they dropped the opener of the Subway Series 4-2.

On Tuesday, Jacob deGrom and the Mets will look to even up the series and take the final game at Yankee Stadium before they host two games at Citi Field.

“We always talk and we have for a long time: When you’re the No. 1 guy, you step up,” Collins said. “Jacob deGrom has done that all season. This stage, he’s used to this stage. He’s pitched in big games in the playoffs, pitched in huge games in the World Series, pitched in the All-Star Game. If there’s one guy you feel comfortable with on this stage, it would be him.”