Dobbins has a long history of injuries but he returned and played well.
Chubb has two catastrophic knee injuries. The last one many people thought he wouldn't return from. Like
@dennehy said, he also already had a lot of wear and tear. While he certainly probably returned too soon, he looked like the worst rb in the league and didn't improve over his time playing then got hurt again.
Sure we could hope he will return to form (seems pretty unlikely). If they do that it better be no guaranteed money because he shouldn't make the team if he is anything like he was last year. In that instance I am fine with the chance. I'd just prefer Dobbins because I saw with my own eyes last year that he can still play football at a high level.