Who are the Worst man utd signings?

Answer 1

In the Ferguson era, here are a few, there are a lot more:
Goalkeepers: Massimo Taibi, Ricardo, Barthez, Bosnich
Defenders: William Prunier, Laurent Blanc, Paul Parker, Mikael Silvestre
Midfielders: Jordi Cruyff, Kleberson, Jesper Blomqvist, Eric Djemba Djemba, Juan Veron,
Karel Poborsky
Attackers: Dong Fang Zhou, Diego Forlan, David Bellion

Some have gone onto become good or decent players after leaving Man Utd, and were perhaps bought too early, or were not given a chance, eg. Forlan and Poborsky, with Veron doing well back in Argentina.