I'm not going to insult Cobb's intelligence on any play or "run him out of town" as many people do when players aren't doing well, but as a fan I can only suggest that the team take a good look at his play and determine whether or not this is the player we want to have handling the running back role.
I don't really see the same player I saw last year, and the four mistakes he made were extremely costly. The tapes may reveal more mistakes, and other plays he made that were exceptionally well. He may have had good total yards, but the OLine was dominating at times.
Fumbles will happen, although they need to be avoided particularly in the red zone, and after the first down has been gained. Going out of bounds was probably just a quick misjudgement. The 1 yard run on 3rd down is probably the play that I really didn't like, because from what I could see there was some space to gain the yardage, and Cobb lacked the speed and power to make the play. On that play Cobb got stood up short of the yardage, not what you want to see happening to your primary running back. Every time the team is second and short I cringe because we don't have a running game to make the play, largely because Cobb does not have the speed or power required.
IMO, if we want to play with the top 3 in this league the team needs to take a look at some other options. If they can find someone better who can pick up the playbook and resume his role, they need to consider a change. There are only 6 games left in the season before mistakes like this will be even more costly. If they can't find a replacement, they need to find other options for short yardage plays.