pretty sure it's not a long standing underlying issue.
I'm not saying he's had it constantly, but he seems susceptible.
I saw here that he also had an adductor injury and a thigh injury in 15/16 season at Salzburg. A thigh injury the season before he joined us from Liepzieg. And then had that strange injury for us away to Napoli where he went off holding his hip and back, and went to a local hospital.
In the wolves FA cup game he seemed to have hip trouble (in the middle of being kicked around) but I can't remember his injury in March. It kept him out for the best part of a month - don't think it was ever disclosd, but wouldn't be a shock if it was hip/groin related especially given what happened at Barca later in the season.