Valencia was one of the best wingers on Europe for those two and a half season. He was our best player on 2011-2012 and one of our best players on 2009-2010. On 2010-2011 he was injured for more than half of the season but after his injury he immediately became one of our most important players displacing the player of the season, Nani. We couldn't find a player as good as Ronaldo, but Valencia was a great player and I doubt that at that time we could have done better.
We didn't replace Tevez with Owen, we added Owen just for the numbers. We already had t good strikers, one of them regarded back then as the best/one of the best strikers in the world and the other was our highest transfer ever. Tevez was already a bench-warmer and scored only 5 goals in the league. That summer wasn't that bad.
The problem was later when it became clear that Hargreaves wasn't ever going to be fit and Anderson wasn't ever going to become better than shit but we still hoped that a miracle will happen and didn't sing someone to replace Scholes. And then we signed Young to replace Giggs and signed Smalling and Jones to replace Ferdinand and Vidic. While it is too early to talk for the last two, it is clear that the other replacements didn't go as good as we expected. Only De Gea was a great replacement.