The problem Alonso had is a set down a phenomenal debut season which immediately raised the expectations and pressure on him to impress almost instantly. During his time here I think he went from 'world-class' to 'very good', to 'pretty good', and then back up to world-class in his final year. I'm pretty confident when I say had he steadily improved we wouldn't be talking about 'two poor years'. I also do not buy into the idea that he was ever "poor" for us, at least not for any period of time worth mentioning.
But I do sympathise with Rafa for thinking about selling him, because our midfield didn't seem to be functioning as well as it should on paper. It wasn't functioning because our side had become heavily reliant on Alonso's brilliant passing to unlock defences, and so when he wasn't quite so brilliant we didn't have a plan B. We never were a side with a great deal of movement - bar Gerrard - and we didn't have the wide players good enough to cut in and play quick, incisive balls to link the midfield and attack, which is what we desperately needed when Alonso wasn't bang on the money. I liken it to American Football - if your QB is having a bad spell, you find other ways of getting the ball over the line. We should have tried something similar.
I don't forgive Rafa for the way he hawked Xabi around Europe, though. That leaves a bitter taste in the mouth, and Xabi did deserve better.