There is some bad luck here, but seasons go wrong for other reasons besides random injuries, and in the case of Lamar's injury the organizational belief that Faalele, and to a lesser extent Vorhees, are starting guards for an offense that is supposed to be elite, contributed, and that is troublesome. Look at film, and its clear neither of those players are good in space, and don't handle the blitz well. Lamar probably got hurt earlier in the game and gutted it out as long as he could- there were a couple runs where he didn't look like himself, and other times he didn't run when he could have.
Ben Cleveland is a better guard but is forever in Harbaugh's doghouse. Time to tank for a new coach lol.