There's much more to managing a team than picking some players, every Villa fan can do that on paper. However, he should have known if players were tired, out of shape, their mindset, etc. Look at Brendan Rogers - he's rested some players, he's given his bench some playing time, he's kept his better performers fresh and sharp, all that despite not many Swansea fans willing to put the 11 he actually did at Villa Park. Still, he got 4 points from two very difficult games. McLeish should have known the players, or some of them, were not up for this Swansea game, that's his job - to know his players better than any fan who'd just put 11 names on a team sheet.
On a side note, on McLeish, he's going to do just enough to keep his position, isn't he? He'll win a game here and there that'd keep us in mid table because we've got a vastly superior squad to some of our rivals, but he'll never be able to make a run of wins, he'd be doing just enough to keep his job.