My hope is he'll be motivated enough this season to show himself on world's stage again after his misery at United. Flick's system will benefit him as well. Then he'll be a good help to rotate and keep Rapha, Lamine and Lewy fit enough for the season.
But under no circumstances should we trigger the buy-out clause next season, no matter how good he'll perform here, because he's exactly that kind of player who will suck again as soon as he loses his motivation and I doubt he'll be ready to reduce his salary for more than one year anyways. Use him for the season, as a LW is badly needed, but don't take over United's problem.