I wouldn't say it was a disaster signing, that's harsh, he played a big part in the 2011 pl title. Ultimately Berba will likely reflect that he didn't make the most of his opportunity here but I think Fergie will take some considerable fault for the transfer. He didn't seem to have an idea of what he wanted from him. I think in his first season he tried using him as a number 9 leading the line which doesn't suit Berba at all, think I remember Fergie saying that he used him wrong. Then we tried him in more off a number 10 role but unfortunately he and Rooney never clicked and got in each others way quite a bit, with both dropping off too often, leaving no one in the box.
In some ways though I think we signed Berba too early. The team when we got him didn't really suit him. There weren't enough players who were getting beyond the striker and so we couldn't make use of Berba dropping off and playing people in. Now we have two strikers in Hernandez and Welbeck, neither who'm he's really played with but who suit him much more as a partner than Rooney. Ando and Clev have come on/emerged and both are players who try and get beyond the strikers whenever they can. Nani has upped his game in the last two seasons, Valencia has become more assured about getting in to the box and linking up.
There was a lot of talk about how Berba didn't fit in to the new style because he wasn't fast and that's partly why Ferige preferred Welbeck/Hernandez etc, but if you look at the signing off RVP, that doesn't really fit in. Pass and move, isn't about speed, you have to be mobile but not necessarily fast, the ball can move faster than any player. Berba has always shown that he's is extremely comfortable with that sort of game and in fact you can see that much of his frustrations came from the overall staticness off the players. There wasn't enough movement.
That's not to say that Berba isn't to blame at all. He can definitely be accused of not really showing enough fight often enough and as being quite wasteful in front of goal, although I think he was very unlucky at times. But I wouldn't say his transfer was a disaster. It hasn't worked out as well as it could and we've not really seen the best from a player with undoubted talent and personally I'm genuinely sad about that because he could have been real quality for us and he comes across as a good guy. Like I said in some ways I think we signed him before we could really use him properly.