The injury, revealed in an MRI performed in New York on Tuesday, is separate from the right rotator cuff inflammation he originally was diagnosed with during spring training.
Since the lat strain didn't present itself on the MRI that Severino had after he was diagnosed with the rotator cuff inflammation, it is believed he might have developed the lat strain at some point during the rehab throwing program he has been under the past two weeks.
While out for the next six weeks, Severino won't throw at all, the team said.
"I don't know if 'relief' is the right word, but a little bit like, 'OK, now we know what it is,'" manager Aaron Boone said Tuesday after the Yankees' 6-3 loss to the Houston Astros. "A little relief that it's not going to be a surgery thing.
"Obviously, it's going to take a little more time now, but a little comfort in knowing this is what it is, it appears to be treatable; just got to take some time, and hopefully we'll get a healthy, strong and fresh Sevy back for a good portion of the season."
Boone said Monday that the Yankees were sending Severino from their facility in Tampa, Florida, to New York in order to undergo a reevaluation by head team physician Dr. Chris Ahmad.
In recent days, Severino hadn't been feeling pain, Boone said. But the team's ace also hadn't been feeling comfortable enough in his rehab to graduate to getting back on the mound. Since his throwing program resumed at the end of March after a three-week stint of inactivity, Severino had done only some long-toss and had advanced to throwing from about 130 feet off flat ground.
According to Boone, Severino was having some of his best rehab work over the weekend.
Boone also reported that Severino "didn't feel as great" on Monday after playing catch in Tampa, which led to the decision to send him for more testing.
Severino has not pitched off a mound since experiencing shoulder discomfort following a warm-up pitch before his first spring training start March 5. The Yankees initially shut him down for three weeks and scratched him from his Opening Day start. They had hoped he would be able to rejoin the rotation by mid-April, but general manager Brian Cashman later pushed that to around May 1.
Clearly, he won't return until after that point now.
About two hours before the findings of the MRI were announced, Boone said he believed in the Yankees' current pitchers in the event Severino's news wouldn't be too favorable.
"We've already seen a lot of quality pitchers step up for us," Boone said. "We've seen some young pitchers take more steps for us. So yeah, we feel like we're equipped to handle a lot of things."
Severino went 19-8 with a 3.39 ERA last season. Earlier this spring, he signed a four-year, $40 million contract extension.
Following Tuesday's game, the Yankees optioned one of Severino's early-season replacements, Jonathan Loaisiga. The 23-year-old struggled in his start in Houston on Tuesday, allowing two runs and four hits on 71 pitches in just three innings of work.
"You're seeing a young, talented pitcher that flashes it so much, and it's just sometimes a learning curve at the big league level," Boone said. "Sometimes, it's tough to make that final step. But everyone watching him can see how close he is and can see how dominant the stuff can be."
Prior to this start, Loaisiga was 2-0 with a 0.00 ERA in career starts in which he had pitched five or more innings. In the three career starts he had in which he had not made it past the fifth, Loaisiga had gone 0-0 with a 5.63 ERA.
The Yankees haven't yet named a corresponding move for Loaisiga's demotion. It's possible they will add a reliever to help their struggling bullpen for a couple of days before making another move that brings 19-year veteran CC Sabathia off the injured list and back onto the active roster.
Sabathia is expected to make his first start of the season back in New York this weekend.
One potential call-up with Severino still shelved is veteran Gio Gonzalez. The lefty had a strong outing at Triple-A on Tuesday, recording 10 strikeouts and allowing just three hits in six innings. After signing with the Yankees late in spring training, Gonzalez was sent to Triple-A to begin the season.