As it stands today, my worries are mainly that he will be playing but at increased risk the first few weeks of nagging hammies or muscle strains because he didn't get time to get in football shape.
The longer the situation seems to stay the same, the more I'll worry he's going to miss time from the back itself. Right now, I'm going with what Jene has said and looking at as week to week for now.
By the way, outside of the injury thing, I think the biggest question about Foster and Tate's success this year is very much what it was going into last year... how will the right side of the O-line perform. Foster's 2 greatest attributes are his vision... and then once past the line his size and speed tend to cause defenders to misjudge and take bad angles on him, where his little dead leg juke is really effective. I've seen people comment about his YPC dropping and saying Foster looked different. I'm not sure what they were seeing. Last year I didn't see a lot of Foster's ability change, but the sheer number of holes weren't there that he could cut and go and get to the second level where his best traits equal extra yards over other backs. There were a lot more guys in the backfield he had to dodge before he could gain positive yards. I'd expect the situation there to be a little better this year just from maturation of those young players on the right side, and Brooks at right guard in particular looks like he may be a good one. Right tackle is still a big question mark. I also think the addition of Greg Jones at FB may help the tailback more than when it was James Casey. Casey is a better receiving option, but not as good of a blocker. But I don't expect a rebound back to 2011 because the line isn't as good. I won't be surprised at all if Foster's YPC goes back up somewhere in between, but that fantasy value is still driven a lot by touchdowns and number of touches.