I don't think this is true, Sancho already looked good at Dortmund, good enough to get an England call up before Haaland even arrived at the club. I remember him looking great against Bayern Munich in 2018, and Sancho had been one of Dortmund's better players for like 18 months before Haaland arrived.
It's clear going wrong for him at Man Utd. Hard to put finger on it, he never looked this slow at Dortmund. He used to run at defenders in the Bundesliga, something he hardly ever seems to do in the Premier League. He has no spark, no confidence in the Premier League, something he never seemed to lack in the Bundesliga. He needs to do better, as I do kind of think there's a player in there, but I am rapidly losing faith, as he just doesn't seem to be kicking on and improving. He needs to be more creative, he's playing for Utd pretty much like a nothing player in possession, as he doesn't really even try to create anything, and takes the safe option backwards most of the time. The one area he has proved useful is he is composed in front of goal when he gets a chance, and can operate in tight spaces, but he needs to offer way more than he currently is.