He'd start ahead of Shaw depending on opponent IMO, or give us the flexibility for both when we go 3-5-2 and actually using a quality attacking fullback on the left side. Shaw is good on the ball in transition but useless in the final 3rd. Reguillon is very decent there (as he showed against us), so would give us far more composure/delivery into the box which is desperately needed. Shaw is a good player, but we can't have both fullbacks so inept in the final 3rd, it hurts us a lot.
Option of:
Rashford Martial
Bruno
Reguillon VdB Pogba AWB
Shaw Maguire Lindelof
DDG
Or
Rashford Martial Greenwood
Pogba Bruno
VdB
Reg Maguire Lindelof AWB
Ddg
Of course though the big thing would be having depth and tactical flexibility now. Same with Van de Beek. Williams should convert to a right back to compete with Wan Bissaka while being cover for both sides as Shaw is injury prone, and then Laird can be added push to give us a group of 6. But definitely need a Reguillon level attacking left back in there.