After watching his sometimes mercurial point guard, Rajon Rondo, dismiss conventional wisdom and excruciating pain and return to the court Saturday night 20 minutes after dislocating his left elbow, Celtics coach Doc Rivers hesitated little yesterday in offering an injury report on Rondo. Hours before Rondo’s MRI came back, Rivers told reporters at the Sports Authority Training Center that the guard would play in tonight’s Game 4 of the Eastern Conference semifinals against the Heat.