I've barely watched him play, but it looks like he's been a left winger since Amorim left Sporting (and, according to Romano, that's the position Chelsea are buying him for - I doubt he'd be much good as a fullback in a back 4) - it's quite possible he sees himself as a winger long-term (rather than a wingback) and that swung things with him and Chelsea, while Chelsea were happy to leave him at Sporting until summer 2026, while United would have needed him to join this summer. In short, Chelsea were offering things to the player and to Sporting that United were never going to want to offer, so this was never really a goer.
Personally, I'd rather United looked for someone with a bit more physicality at RWB - Quenda looks quite capable in 1v1s, which is also something we need, but he's a 5ft7" skinny 17-year-old who is not going to do much to bring up the floor of this United squad's physicality, which is badly needed this summer.