I disagree with your assumptions. Take Ched. He was largely accepted as garbage for the couple of years we had him prior to Wilson's tenure. Another misfit. What Wilson did was get the best out of what we had. At the moment it looks as if Weir is not doing the same.
Now before people interpret this as an anti Weir post, it isn't because what we don't know, is, if McDonald had not gone or rather if the whole transfer thing had not happened, would we have done better than we have? Of course we can't answer that or prove it either way. If the perception is that we would have done better, then you might reasonably argue that Weir was 'torpedoed' and is now in damage control/limitation and is working on plan B, which to be fair might need a bit of time (but how much time do you give him). If the answer is no then it is not unreasnable to conclude that Weir's ability might not be as good as people expect.
Take DB and NW. They both stepped in when United were going tits up. NW in the autumn, DB at the end of Jan. NW saved us easily from relegation, DB didn't but we came back strong the next season. On that basis and given that in theory by May 2015 we could be anywhere from being promoted to the Prem or relegated to the Conference you have to say how long do I give this guy.
I would say if he hasn't secured some semblance of order by the end of October - 3 months in charge) he is not the right man for United. It would then be a judgement of stick or twist. If you stick and he takes us down what do you do then? Our record at surviving relegation scraps with new managers isn't good.
Of course we may then be 8th, sorted out the wobbles and a reasonable season may ensue, but at the moment 5 games without a win for one of the promotion favourites doesn't look good. The last club I recall doing so badly was Keane at Ipswich, who were ' a favourite' and it proved rather unsuccessful for both parties.