It's not that I consider every manager we've had has been incapable of recognising if a player would prove a useful asset or not. I think our recent managers have made decisions that they believed would help us move forward, the trouble is, most United supporters can soon tell if a player was a worthwhile acquisition. We've fallen short when it comes to buying enough players of the necessary quality to impose ourselves on other teams, and by that I include someone who can pass a ball, or someone whose job is to act as enforcer, or that most glaring of omissions, a brick shit-house of a centre-half who'll command his area and instruct his fellow defenders to do their jobs properly. These are just a few examples, and of course we need two, possibly three quality central defenders for starters, but I suspect you'll know as well as I do what positions need strengthening.
So the answer to your question is no, not all United managers have been poor judges of a player's ability, but our recent acquisitions have, in the main, been underwhelming.