You've missed the really obvious reason for Valencia's poor early season form. Which is odd, seeing as it's contained within your post.
Once Valencia stopped having to cover for injured fullbacks he hit a decent run of form which lasted almost the whole season, despite an unfortunately timed injury. Which is a consistency that neither of our other wingers achieved.
Well, not really. I like Valencia, and IMO he, Carrick and Evans, are our players of the season, but his start of the season was really poor, and I am not counting the game where he played as a fullback. Let's do a little review:
vs benfica(His start of the season ) -where he was starter as a winger, and like rest of our team, had quiet game.
vs Chelsea - benched, introduced later as a fullback
vs Leeds - he was starter as a winger, but tbh I don't remember his performance. Let's say he had a good game.
vs stoke - he played as a fullback,
vs Basel - he played as a winger and he didn't have his best game, Nani replaced him later.
vs Norwich -played as a fullback.
vs liverpool - where he was benched again, but in that game most of our first choices were dropped too. Carrick, Rooney, Nani didn't start that game too. But, both Nani and Rooney were introduced later, but Valencia stayed on the bench for whole game.
vs Otelul - He played as a winger, not really good game, he was substituted in 71st minute.
vs City - He was at the bench, didn't feature at all
vs Aldershot - played whole game and scored, had a good game
vs Everton - he was on the bench once again, introduced later
vs Otelul - played as a winger, and put a good performance
vs Sunderland - spent game the bench
vs Swansea- Spent most of the game on the bench, introduced in 82'.
vs Benfica - Starter, had a good game
vs Newcastle - bench, didn't play
vs Crystal Palace - Played as a winger, our only threat in that game, but considering the lineup it was expected, but he didn't have his best game, once again.
vs Aston Villa - bench, but he was introduced early instead of injured Hernandez, had a solid game, nothing special.
vs Basel - he wasn't starter once again, watched the game from the bench
vs Wolves - probably his best game from start of the season, started as a winger, picked up 3 asists.
This game against Wolves was probably game where his season really "started", after that game, he was in great form untill rest of the season.
Before that game his form was nowhere near his best, it's fair to say he underperformed. He did play some of the games as a fullback, but he played more games as a winger and in those games he wasn't puting good performances, prove for that is that he was often benched, there were games where he didn't feature at all and he was fit.
Game against Benfica(his returning after injury) was played 14.9.2011, and the game against Wolves was played 10.12.2011., that's 3 months.
