Wasn't at the game yesterday - was at Murryfield - but I'm assuming that Pearson was left midfield and if so should have been helping protect the left back, which in turn would mean that he was partly responsible for leaving Chalmers (yes a poor player) exposed. This is not something I'd be expecting of a senior player.
So in short why play someone at LB who can't be arsed helping to cover LB?
If he wasn't left midfield then that's different - not a lot though.