Speculating but confident in narrowing down to 3 suspects:
Green: It's been well-documented that Gerald is auditioning for several gangs in the greater Boston area. Depending upon affiliation, he may stand to benefit from inserting his elbow into the head of the new local star.
Young: While he won the battle with RJ, the war persists. Young's on-court awareness falls in the borderline impaired range. He can't get out of his own way, so it may be a bit much for us to expect him to get out of big Al's.
Smart: Kid was born to be a wrestler, one of the greats even, and has successfully transformed himself into a "basketball player." But his "elbow to head, pin to mat" tendencies are deeply-ingrained -- if it wasn't him, it will be soon enough.