I would love to sign Ryan Sessegnon as a back up understudy to Shaw. Originally was a LB, but then Fulham saw his attacking quality and put him at LW, LWB, and LB. His defense wasn't all that great, but considering his young age, it's not that surprising.
At a club where the attacking options are limited, it would be a waste for Sessegnon to be a LB as you would want him attacking up front where he can do most damage. For a club the size of United, you aim for something higher and with even more elite technical ability like Sancho. For a LB, his technique would shine more because he would now be compared to LBs. His attacking ability would shine more because his attacking ability would now be compared to LBs. Not many LBs could do what he did in the Championship as a LW. That shows his attacking ability is superior.
Clearly he isn't good enough defensively right now. That's why he would make for a good back up to Shaw when United are chasing a match. He's from the same 2000 born generation as Gomes and Sancho.
Fulham are getting relegated and he's had a "down" season. In other words, his price has gone down even though it's no surprise an 18 year old didn't set the PL world on fire. LB is not a priority, but the time to buy is now with Fulham getting relegated. Spurs were rumored to be interested in him so he could cost a lot more down the line.