He's been disappointing, but at 27 when he signed I expected him to be fairly useful as a squad player. He's one of those players that seemed to age very quickly and now plays like he's years older than he is.
He was a perfectly acceptable signing for the sort of position we were in when he signed. Just finished 14th in the premier league, he was 27, a Belgian international. Wages were a bit high, but not "dangerously" so. If we'd been a club with aspirations for "top half" finishes, he'd probably have played quite a lot over the last few years, but the reality is our transformation under Unai was so significant, and SO rapid, that he was just left behind. Some players were capable of taking their game to the next level under Unai, Donk was not (my guess, as others have suggested, is that it's his lack of mobility that prevents Unai wanting him in the squad). Not his fault, and I don't blame him for not moving for less money.
It's unlikely anyone will touch him while he's still on £90k a week with us, but if we came to an agreement to release him from his final year, I imagine he'd have his pick of a dozen or more top flight clubs across Europe - each of which would happily pay him the 20-30k a week he's probably worth these days. I dare say he could also do a job for a newly promoted side to, one that is going to be playing with the backs to the wall a lot, and isn't going to be too expansive or dynamic on the break - but none of them is going to pay any serious cash for him.