United still need to solve a problem position on the right side of their attack - and if they land Haaland, they are unlikely to be able to afford Sancho as well.
And that scenario could see them to back in for Watford winger Sarr - after the Championship club's former technical director said the two clubs were close to a deal last summer.
The Hornets were open to selling Sarr following their relegation to the Championship, but wanted £40m for the wideman - and the deal nearly happened, according to Filippo Giraldi.
"We were extremely close, I was personally negotiating with them," he said on the Here We Go podcast.
"We weren't able to reach an agreement. And I think Ismaila is very young, he will arrive for sure.
"You see fast wingers becoming more and more important in physical leagues like the Premier League.
Sarr, 23, has scored nine goals and posted nine assists this season to help fire Watford up to second place in the Championship.