What happened to him? Wasn't he one of the best defensive midfield prospects in world football?
Tuchel's dominating football was perfect for Weigl, highlighted his strengths and hid his weaknesses. But the first cracks already appeared during Tuchel's and Weigl's second season at Dortmund, when the team couldn't control games nearly as well anymore after losing Mkhitaryan, Gündogan and Hummels in one summer, perhaps also because of Dembele's high risk/high reward style.
Bosz likes to defend with the ball, but he also wants his CMs to push higher up and that didn't work at all with Weigl to the point where it seemed like he was ignoring instructions and Sahin made a comeback as a regular starter, because for all his injuries and slowness he was/is happy to interpret the role in a more attacking fashion.
Under Stöger, who finished that season as a care-taker, the team had even less control over matches, Weigl started most of the time, but everything was a mess the team had less and less confidence and the midfield was often wide open, making someone as slow and lanky as Weigl looking lost..
So the following summer Dortmund's management felt they needed a rebuild and that their midfield was too often pushed over too easily, they signed Witsel/Delaney. The Belgian is just the more complete player and with all his experience he apparently also became a bit of a team leader immediately. So that was that for Weigl as far as being the midfield anchor. He picked up some minutes here and there when Witsel was injured and established himself as the team's 3rd/4th CB, but bottom line is that his departure was only a matter of time after Witsel's first couple of games.