Yea, Ferguson doesnt make poor signings. Djemba-Djemba, Kleberson, world-beaters turned out of depth Veron and Blanc, Forlan (he started some matches at EPL and at Cup competitions, come on, even you Man Utd fans say he wouldnt score in a brothel, now he hasnt been given a opportunity? ha!), and i'm saying that from the top of my mind.
Benitez signed Reina who's been our starter for several years already, he signed Torres, Alonso, Arbeloa (want it or not, he's a Spain international and he's good enough for the EPL), Benayoun who's a good performer for us, Skrtel (quality defender), Mascherano, Dirk Kuyt, so i dont really see where you're coming from. You're talking about the signings that have sucked, but they're youngsters for christ sake. Lucas, Babel, Ngog and co. cant be considered terrible signings, all of them have shown potential and quality (Ngog not so much but well...). At the moment there's a lot of fringe players at the club, yes, but they have potential.
They're not supposed to be world beaters right now. We're talking about squad depth here. The only signings that can be considered as failures at the moment are Voronin and Cavalieri, even tho the latter has lots of potential and you cant blame a goalkeeper for screwing up when he plays once for one-two months. Riera is so-so, ok. Anyway, hardly everyone sucks huh?
Take into account that Man Utd have more money at their disposal for signings and usually get their top targets whilst we have to settle for second/third option players because we have insufficient funds.
