Mandzukic is an asset but only if his mindset settles to be a regular backup. That's going to be his position in any top club he wants to go anyway.
So it's up to him. He can be the backup in the best clubs, be the starter in lower level clubs or forget about competitive football and become filthy rich.
He strikes me as the kind of guy that would choose options n. 2 or 3 before option n.1, as he showed a few times.
But the way it has been till now, he's not doing any good to himself or to the club. He's gotta decide. His drive has been down, and that's basically the difference between him and an average striker. If he can't motivate himself, he really should look at his options.