They're already on a 9 game unbeaten streak in the league and they've already significantly closed the gap between themselves and 4th place. Now they've signed the best forward in the league plus Zakaria who will improve their midfield.
It might take them time to adjust and all that, but at the same time it might not. Plenty of new players, especially those from within the league, often hit the ground running. Regardless, it's not like Vlahovic is going to forget how to kick a ball all of a sudden. Any chances fall to him and you can bet he'll be deadlier than Morata.
Looking at their competition, Atalanta have gone through an injury crisis while Napoli are a massive bottlejob just waiting to happen. Milan may falter too.
I've been saying this even when juve were in 9th place or whatever - they will make top 4. While it's just my personal opinion, after the mercato they've had, I think their chances have gone from 90% to 95%.