Well that was sort of to be expected at this stage. It would be a lot easier to "forgive" Perez if we actually saw them both on around the same piece of tarmac at some point in a race, at least. Being beaten by Jenson - OK, never being in sight of him - we have a problem.
There's still plenty of time for Perez to turn it around though, I'm sure McLaren won't get rid of him before the end of the season at the earliest and only then if they really think there's no hope of him getting up to speed.
Also, I'm quite surprised people are suggesting Di Resta, he got absolutely trounced by Hulkenberg in the second half of last year. Granted he's shown promise in his first season and the first half of last year but Perez got podiums and beat his more experienced team mate in the WDC. Hulkenberg would have been the obvious choice but hadn't he already signed for Sauber? Apart from him you're just left with guys like Kovalainen, Kobayashi and Glock or someone from within McLaren like Paffett or a young unproven driver from elsewhere like Bianchi. I don't think any of those are particularly good options for various reasons.
I think McLaren made the right decision based on the evidence and the options available to them at the time. Well, their mistake was not doing enough to keep Hamilton and the fact that Hamilton is now beating them in a car that was at the back of the midfield for much of last season tells you why he left.
So after a few edits this post has got quite a bit longer!