His skillset.
A LWB is of course a halfway point between a left back and a left winger. In a system that has abolished wing backs - he’d need to try and find a home in one of the two roles. As a wing back - he showed very poor attacking quality for a year, and constantly found himself receiving the ball in dangerous positions and not demonstrating the quality to do anything with it. I think naturally, the observation would not be that he should be given greater attacking responsibility.
Regarding playing left back, it’s just simply easier to learn. It’s not that he’d necessarily shown himself to be a brilliant defender, however - he has attributes that could make him an effective left back in the modern game. He can run all day, he’s fairly quick, and has a good physique. Playing as a forward for a club of United’s level requires a high level of God-given talent, which to me - he clearly doesn’t have. He has not shown the footballing ability to say he can be relied upon to make a consistent attacking difference at the highest level of football.
Full back is simply somewhere players like him typically end up. More talented wingers like Young and Valencia ended up there eventually because they were simply not, or no longer, good enough to make regular attacking difference. Aaron Wan-Bissaka made the move earlier, as have several others, like Ashley Cole and Patrice Evra. The tools Dorgu DOES have offensively are good supplementary tools from an attacking perspective, but not enough to be the primary threat.
I don’t know why people have chosen to write off his year at wing back, which evidenced poor attacking quality in dangerous positions and explain it as him being ‘out of position’ or something, and then advocate for him to be in those same positions he continually fecked up. We’re asking him to do what top teams are asking of Doue, Kvara, Vini etc. I see him as a player like Frimpong at Liverpool, who is clearly never in a million years going to be good enough to simply be a Liverpool forward, despite him being a ‘can play there’. I’d be overjoyed if Liverpool genuinely considered him a key forward. Frimpong was bought as a right back, having played at wing back. He also played as a winger for Leverkusen at times. But he’s a long way from being a Liverpool winger, and they’d have never signed him to play as a winger. He’s very similar to Dorgu.