Tadic isn’t that great either, bar his 18/19 performances in CL.
Tadic is a good player. For sure not a world-class, but in terms of quality and what he brings to the game he makes a difference quite often. Sure, that 2018-19 is his most memorable season and especially that game at the Bernabeu, but he's been consistent ever since both for Ajax and our national team.
If he's used in a good system and allowed to have some freedom/control over how he plays, he produces good and consistent performances.
I was solely comparing him and Kostic; Tadic is for sure a level above. But on the other hand, they don't play in the same position either.
Anyway, if Filip comes in I think he will be a good rotational player for us. Someone that can understand and provide good quality balls from the flank for Dusan.