The difference is fairly obvious, I would have thought.
Amorim was one of the most highly rated young coaches in Europe, someone the club identified as being qualified to be United manager, tied themselves to with a permanent contract, and then started briefing from day one would need a long time to rebuild.
Carrick wasn't one of the most highly rated young coaches in Europe, wasn't someone anyone thought was qualified to be United manager before this interim spell, isn't someone we're in any way tied to beyond the end of this season, and whose future is obviously entirely dependent on short term results and performances.
No shit the former context lends itself to calls for patience more than the latter.
If Amorim had been hired in the same conditions as Carrick (interim manager, contract until the end of the season, no expectation that he was actually qualified to be United manager, entirely short-term focus), he would have been bombed out of here within weeks.