I was surprise to see him playing this match, he was visibly discomfort while playing last two ODIs...He was not seems to be not fit in last AUS test, yet still played.
Why this is happening is hard to speculate, on his part, he is under pressure because he has not delivered the goods most expected him to do. Because of that, he has been playing with niggles in some cases more than just niggles. He feels (or maybe given warnings) that he is one match away from been dropped for good, that could be cause of him hiding injuries...
Team management and Captains want to play him because he is still the best new ball bowler (despite not getting much wickets), he is the one who control the game, beats the bat, brings down the run rate. Pakistan has played 10+ test in 8 months, most of those tests were fast bowling heavy loads one. Forget about Amir, Yasir also got injured because they both were suffering the majority of the load...Looks like nobody cares or understand those things, we could clearly see in ODI series or in AUS test series that Amir is not fit, nobody talks about it. One day we will hear that he is out because of back or shoulder injury, could be for long period, very disappointing stuff from management.
People say, those 12/15 catches is part of game, that means they bowled 100+ extra overs, and could have allowed us win 2 more matches not to mention, could have brought back our main bowler to limelight. Little things matter a whole lot in Cricket, specially when your team is not full of star and is on the road.
Management and probably Amir himself is taking a dangerous route, we don't have much bench strength. Amir's Career could get a serious road block. He is going through too much workload since his comeback.
BTW: County Contract is not only valuable from monetary perspective, its real value is experience. Wasim was a very different bowler after couple of county seasons. County is the best place to master swing, not many bowlers get that opportunity, Amir got it because of his 2010 season. To me thats more important for his development than playing test on dead wickets or ODIs. He need this season to get back his mojo, its important investment for Pakistan as well.