Mitrovic has played 3 full seasons in the Prem. The first time he scored 9, then he got 11, and last time out he got 3.
They obviously aren't great numbers and the 3 he scored last time was poor but considering he's been relegated with poor teams in all of those seasons I don't think the first two are bad by any means. He would have only been quite young when he first went to Newcastle too.
I think if he goes down with Fulham again someone better might take a shot on him if the price is right. Maybe go to another top flight league abroad where the standard isn't as good as the Prem. He'd probably do a job for some sides fighting for Europe in the right set up.