Well.. there were also Mike Hanke and Igor de Camargo who did relatively well for him at Gladbach. Though Luuk de Jong was a massive flop.
At the moment Favre uses two dribblers (Sancho/Hazard) out wide and Reus as a #10/#9 hybrid who links up play centrally and then drives forward into the striker role. I don't see him playing either role. So for him to start matches Favre would need to deviate from a system that has served him very well (results against Leipzig and Hoffenheim aside, which were both unfortunate imho) and it's hard to speculate what that might look like.
I think the second part of your post is probably right though. Favre had already more than one foot out the door after the fairly disappointing October/November matches and there is no credit left if the team's form dips again. I reckon there's a 50/50 chance at best that he will survive the summer and thus it makes little sense to fine tune the squad around his needs, especially when you can land such a high profile transfer.