Normally I'd agree with Robben. First season, in and out of the team in a much tougher league than one where he's just bossed it, it's going to affect anyone.
However if the reports of his bad attitude are true then there's not a lot we can do. If someone doesn't want to learn and improve for the sake of both the team and their career then you have to cut your ties.
I just hope he is actually trying hard and isn't going to let a tough debut year get to him. There's clearly some talent there, we just need to work out how best to use it, all whilst he does too.
From what you can glean from his attitude and the type of person he seems to be (massively reaching), you can imagine Martial coming in and immediately lighting up the place was a dent to his ego and he's suffered with that all year.