If you're in close enough proximity to get burned, that's tight enough and Saka would more than welcome it and fancy his chances. Going out to such a tricky wide-man by yourself and then leaving space behind you with willing runners eager to breach it doesn't seem prudent in a game where your midfield is nowhere to be seen - Shaw would've been stuck between a rock and a hard place then, and subsequently blamed if anything was played in behind him.This is sound in theory. However, in practice Shaw was far too passive and didn't engage in any sort of baiting Saka to go either side or attempt to 'get tight'. Getting tight doesn't need to mean getting close enough and possibly getting turned, it can mean at least trying to shield them away or make them hesitate to come forward.
Yes he didn't have much 2nd help and perhaps Martinez was a bit deeper than usual but conceding territory like that so easily doesn't sound like a tactic to me.
When you say shield them away, that's usually because options on the inside are limited otherwise an attacker who has anything about him will actively invite that contest because the absolute worse that can happen is them being dispossessed, whilst for the defender, there's a number of pitfalls (beaten all ends up; cards; played in behind; taken completely out of a play with a simple lay off) that aren't prudent unless the rest of the team are ready and waiting in cover to sweep up.
Conceding territory isn't great, but the alternatives are far worse, at the very least, Saka then has to make and execute the right pass/play, rather than having free reign in open space to then probe the CB and have the easiest of lay offs to onrushing attackers with the fullback barely able to interject should he be behind his man.