Already mired in a difficult streak, the Boston Celtics were dealt a crushing blow Sunday afternoon.

All-Star point guard Rajon Rondo has been ruled out for the rest of the season after tearing the anterior cruciate ligament in his right knee.

If the playoffs were to start today, the Celtics would be seeded eight in the Eastern Conference. The loss of Rondo might mean the Celtics will consider a major deal. Before the injury, the 26-year-old was notching 13.7 points and leading the league in assists with 11.1 per game.

Head coach Doc Rivers will be forced to rely on his veteran guards, though none can be expected to produce like Rondo. Next up on Boston’s depth chart is Leandro Barbosa, but Rivers opted to start guard Courtney Lee on Sunday against the Miami Heat.

Most often used as a scorer, Lee is currently averaging 22.7 minutes per game, and hasn’t been a regular starter on any team since 2010 when he was with the New Jersey Nets.

Rivers could turn to 35-year-old Jason Terry, but he is having a down year, averaging just 9.8 points and 42.6 percent shooting since he left the Dallas Mavericks. Guard Avery Bradley is Boston’s best perimeter defender, but it would be unlikely for Rivers to ask him to run the offense.

Team general manager Danny Ainge could turn to the Celtics D-League affiliate, the Maine Red Claws. Veteran guard Andre Barrett is averaging 8.2 points and 11.2 assists in six games this season, and has four years of NBA experience.

Boston has reportedly been in the trade market for a big man, but were also tied to Orlando Magic guard J.J. Redick. Rondo’s injury could accelerate talks.