And then there were five.

It was only last month, in balmy Port St. Lucie, when the Mets were reveling in their pitching depth, salivating at the young starters who could deliver them to their final goal, and the capable supporting cast that could help sustain them during those long summer months. But things happen pretty quickly once the regular season starts, and in just three short days, a Northeast chill has ripped through the Mets pitching staff.

With Steven Matz down with a strained flexor tendon and Seth Lugo benched thanks to a partial UCL tear that could require surgery, the Mets have once again been reminded that nothing is guaranteed. Not even all that pitching.

“We came out of spring training saying, ‘’We’re seven deep . . . and right now, we’re not. We’re five deep,” Terry Collins said. “Anytime you lose quality starting pitching, you have concerns, and I think obviously. that’s why we’ve got to be more cautious than ever, certainly in the first few weeks of the season, to make sure we don’t overdo it . . . We’ve got to hope a couple of those guys in Triple-A get off to good starts so we’re a little more comfortable with the depth that we have behind these guys.”

Lugo said the initial MRI on his elbow appeared to only show inflammation, but a consultation with a specialist — Dr. David Altchek — revealed the small tear. The hope is that the ligament will respond to platelet rich plasma injections (his first was Tuesday) and that in two weeks, he’ll be able to resume throwing. However, general manager Sandy Alderson said that surgery is possible if Lugo’s elbow doesn’t respond.

Alderson said the imaging confirmed that Lugo’s injury happened recently — which means he may have hurt himself in the World Baseball Classic. The injury may also be the result of an overall increased workload this spring, and both he and Collins said they still believed the WBC was beneficial. Collins did say he wished teams would start training for the tournament earlier to allow for extra rest time.

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“I felt something similar last year. It’s more discomfort than there was this year, so I’m optimistic about that,” Lugo said, in much better spirits than he was when the tear was first detected Monday. “I was pretty concerned at first . . . Looking at it, it’s a small tear and they were talking — a lot of pitchers pitch through tears and some don’t even realize they’ve got tears. I haven’t felt any pain, so I think that’s a good indication that it should be a quick recovery, or at least I’m hoping.”

What’s certain is that no matter what happens to Lugo, the Mets absolutely have to protect the remainder of their pitching staff. Matz’s injury will keep him out a few weeks at least, though Alderson said he didn’t “think [surgery], right now, is in the realm of possibility.” Also, Jeurys Familia continues to serve his 15-game suspension. Jacob deGrom and Matt Harvey are coming back from season-ending surgery, and Zack Wheeler hasn’t pitched in a major-league game since 2014. We’re not even counting Noah Syndergaard’s blister, which has pushed back his next start by one day.

“It’s not really frustration,” Alderson said. “That’s why we had the depth. Unfortunately, we had to use it so early. Hopefully it will present itself again in mid-May. In the meantime, we’ve started to just kind of sniff around for additional depth. It’s an ongoing process, the fact that we have the depth, had the depth. We don’t have terrible concern at the moment, hopefully it will be back.”

The Met’s top-ranked pitching prospect, Freeport’s Justin Dunn, is with Single-A Port St. Lucie and still needs development. The Las Vegas 51s rotation — the Mets’ Triple-A affiliate — is headlined by Sean Gilmartin and righty Ricky Knapp, who’s coming off a strong 2016 campaign in the minors.

“A little bit, yeah,” Alderson said when asked if the Mets plan to be even more sensitive to how they handle their pitchers. “Now we’ve already taken steps to monitor that, from the beginning of the spring training, not just through the spring, but as we get into the season, it’s not about implementing a new plan. It’s adhering to the old plan as closely as we need to.”