If you hire a guy as interim you're explicitly saying that your assessment is that he isn't good enough to be a permanent manager. Under normal circumstances, no one hires an interim manager. It's a made up thing that clubs do only when they have to get rid of someone and they can't figure out who the next guy should be.
The problem is that clubs, and specifically United, confuse short term success as proof that their initial assessment was wrong. Carrick's managerial flaws didn't disappear just because he had 4-5 months of good results.
A long term interim manager (i.e. more than 1-2 games) is a tool that clubs use when they don't have a plan and don't know what they want to do. Well run clubs don't do interim managers because they have a succession plan or a list of candidates they are confident in should it all go wrong with the current guy. United only gave Carrick a 2 year contract which implies that they were hedging their bets at least a little. Then they should already have a list in the safe of potential candidates to go after in case of emergency. But I doubt they do.
The problem is that clubs, and specifically United, confuse short term success as proof that their initial assessment was wrong. Carrick's managerial flaws didn't disappear just because he had 4-5 months of good results.
A long term interim manager (i.e. more than 1-2 games) is a tool that clubs use when they don't have a plan and don't know what they want to do. Well run clubs don't do interim managers because they have a succession plan or a list of candidates they are confident in should it all go wrong with the current guy. United only gave Carrick a 2 year contract which implies that they were hedging their bets at least a little. Then they should already have a list in the safe of potential candidates to go after in case of emergency. But I doubt they do.