Seth Lugo's partially torn UCL likely occurred during the spring, Mets GM Sandy Alderson told reporters Wednesday at Citi Field.
Alderson said doctors told the team the presence of fluid in Lugo's elbow suggested the injury was recent, adding that the Mets are optimistic the injury won't require surgery.
Lugo will rest for roughly two weeks before beginning a throwing program.
Whether or not Lugo needs surgery will depend on the results of the platelet-rich plasma injection he received, and how his throwing program goes.
If Lugo needs Tommy John surgery, the recovery time would be 12-to-18 months.
Lugo was examined by Dr. David Altchek this past weekend, at which point he was diagnosesed with elbow inflammation and told Anthony DiComo of MLB.com that it was a minor issue and he hoped to begin throwing Monday.
Lugo said he was dealing with arm fatigue during his last start, most likely a result of pitching deep in to the World Baseball Classic, manager Terry Collins told reporters Friday morning at Citi Field.
The Mets were 7-1 when Lugo filled in as a starting pitcher last season, during which he also had a 2.65 ERA in nine relief appearances, while striking out 8.5 per nine innings. However, he struggled during his final start of Spring Training, allowing four runs -- including three home runs.
Rafael Montero made the roster in place of Lugo.
Montero, 25, had a 1.77 ERA and 1.13 WHIP with 23 strikeouts in 20 1/3 innings this spring.
Despite being in the organization since 2011, and throwing more than 100 games in the minor leagues, Montero has appeared in just 24 major league games during the last three seasons.
He was sidelined by a shoulder injury during most of 2015, during which Terry Collins made a special trip to St. Lucie late in the season to talk with Montero about his status.
In 20.1 innings, he's 1-0 with a 1.77 ERA in 10 Grapefruit League appearances, during which he has struck out 23 batters and walked just eight.
"He's throwing the ball very well, but we haven't finalized the roster," Terry Collins reiterated Monday after watching Montero throw two scoreless innings against the Braves. "He throws strikes. We always knew he had the command but he kind of pitched to the edge in the big leagues. Now he's using the plate and his stuff is playing."