I'm not sure a new LB is needed.
If Ineos restructure the medical department, I'm expecting Shaw's injury record to improve, but we also need to manage him better...meaning that he shouldn't be playing 60 games a season. One game a week with a better medical department supporting him, and I'm confident we can see the 20/21 and 22/23 Shaw again.
Malacia is also better than most of these names I'm reading in here, barring maybe Gutierrez. There's also nothing to suggest he's an injury prone player. I'm pretty sure he will come good at United.
Martínez could (and maybe should) take up the LCB/LB role that Arteta and Pep have been using lately.
And then we have Amass, who looks like someone who will surely play for the first team one day.
Shaw, Malacia, Martínez and Amass could potentially be the best left back depth on the whole continent in a few years' time, IF the injury situation gets better.
If we do buy someone, I hope it's just a cheap punt similar to the Malacia deal..so in the 15-30m range.