Fullbacks are incredibly important in modern football. Top teams really need athletic, skillful FB's who can cross and offer a genuine attacking threat. City managed to significantly strengthen their squad there whereas we didn't do anything this summer. Walker, Mendy, and Danilo went to City, Semedo went to Barcelona, Aurier went to Spurs (who to be fair lost Walker), Dani Alves went to PSG (who signed two other FB's). Even Chelsea signed a fullback in Zappacosta after winning the title last year and Juventus signed De Sciglio. Real Madrid, who has the best left back in the world, signed Hernandez who is probably good enough to start games in most teams in their league and is only 19.
This window had so many fullbacks moving around because people were realizing how important they are.
We have a bunch of players who can cover these positions but none of them are good enough to be starting regularly unless our ambition is to remain around 4-5th position in the league every year. Shaw is less reliable than Owen Hargreaves. It's surprising that Mourinho let this summer pass by without signing anyone who can play there. It's going to ruin his 2nd season title winning record.