Sisto was unstoppable when FC Midtjylland beat United 2-1 in 2016, that's part of the reason he moved on to a bigger league. He started well in La Liga, and on his best days he really is a top class winger, but the overall performance in Spain has not been good - I completely agree to that.
If we managed to get him up in gear again, he has the ability to be the deciding factor in matches. I just don't see it happening, he has been battling with his mental health, and at the same time it would be a big step up for him to handle the physical side of the Premier League.