If it was a situation where we had made significant improvements in other key positions but decided Sancho wasn't worth the money then (based on what I've seen of him) I'd be quite happy with Sarr. He wouldn't be in any way a transformative signing but he wouldn't really need to be. He'd be more depth in a position where we badly need more depth and would fill out the profile of the squad. And, who knows, maybe more than that.
The problem is we haven't made that significant improvement elsewhere. So any signings we make between now and the close of the transfer window really need to be able to do a lot of heavy lifting over the season. Signing Sancho wouldn't help our other problem positions but (if he proves to be as good as we think) he could have the sort of impact that helps carry the team over its other faults. With the best will in the world, Sarr isn't going to do that.
If we were to end up signing him, I'd be reminded of what Van Gaal said since leaving:
"I didn’t always get the players that I want....I thought always Manchester United can buy every player because they have a lot of power. Seemingly a few players were not reachable for Manchester United. I cannot understand but it was like that.....I was satisfied, because [Di Maria] was a creative player, but I had other players on the list..... I wanted Robert Lewandowski, but when this proved difficult I tried to get Gonzalo Higuain instead. Before I had arrived, I also spoke with the board about Neymar. If you are at United, you have to think big. He was also interesting for the club in terms of selling shirts, and I wanted to have quick wingers. For that reason, I also tried to get Sadio Mane and Riyad Mahrez. Thomas Muller was on my wish list, too, and in central midfield I wanted N’Golo Kante. I even tried to sign James Milner, who was already quite old but very multifunctional and possessed leadership skills. For the defence, I wanted Sergio Ramos and Mats Hummels, because ours wasn’t the strongest in building from the back.....Those were my top targets, but we couldn’t get any of them. I don’t know why, because as the manager I wasn’t involved in any negotiations....After I left, players like Mahrez and Kante ended up at Manchester City and Chelsea, while United couldn’t get them. I found that very odd....."
I suspect Solskjaer might have a similar tale to tell if his time here ends in failure.