It's not really a new injury and it's not serious, but part of the rehab side effects. I read that he has lumbago. As his injury with Arsenal was a hamstring one that's not really surprising, the two are linked.
He wasn't going to play the opening 6 or so rounds, he had little pre-season and played 20 minutes of a game.
What I don't get is why the druids called him up in the first place.