Morgan, the Minutemen's starting shooting guard and their second leading scorer at 13.4 points per game, was injured in Thursday's 70-62 loss at St. Louis. Morgan's right knee buckled as he drove to the hoop and he howled in pain while being tended to by team trainers.
The initial diagnosis was a sprain, but after Morgan was seen by doctors on Tuesday, the tear was discovered. He is expected to have surgery next month.
The Minutemen are 11-4, 1-1 in the Atlantic 10.
Coach Derek Kellogg likely will use senior Freddie Riley in Morgan's place, as he did in Sunday's 77-73 win at Fordham. In that game, Kellogg also used regular point guard Chaz Williams in the off-guard slot with freshman Trey Davis playing the point.
"I feel bad for Jesse, obviously," Kellogg told the Daily Hampshire Gazette. "He's worked really hard to be one of our team leaders and one of our better players. To have an injury like that cost him the rest of his junior year is tough for him. It's tough for our team to a certain extent. We have some other guys that are ready to step up and play for Jesse."
In addition to being an explosive scorer, as evidenced by his 35-point effort against Ohio on Dec. 19, Morgan is UMass' top perimeter defender and a key 3-point threat, which is a big part of the Minutemen's offense.
While Riley has been a solid contributor both offensively and defensively over the past half-dozen games, averaging just more than 11 points, he's been a bench player most of his career. Even if Riley approaches Morgan's production, the loss in depth figures to hinder the aggressive, pressing defense the Minutemen have used with good results the past couple of seasons.
Davis had the best game of his career against Fordham with 12 points and four assists in 21 minutes.
Steve Richards is a regular contributor to ESPNBoston.com