It's always the go-to if United are struggling but it's not much use if the players aren't good enough. There's an aspect of you don't know unless you give them a chance, but the coaches would have a fair idea who is good enough and who isn't.
Borthwich-Jackson a few years ago was actually good when he played but then spent most of his career in the lower leagues for example, plenty of cases like that. Brandon Williams a free agent now? Was a big push for him not that long ago.
Also Garnacho is 20, Yoro is 18 etc are youth.