Same as all the other managers they've had since Moyes, it'll probably go well in the first 50% of Amorim's tenure but then standards will drop again, as they always do, and the slow downward crawl will begin. They might win a big game or two (or even a trophy) but then a switch flicks and they need another big changeover. It's happened to every manager they've had for a decade.
Van Gaal. Finished 4th in his first season, got them back in the Champions League. Next season, they won the FA Cup but they dropped out of the Champions League places again and sacked him.
Mourinho. Finished 6th and won the Europa League and League Cup in his first season, then finished 2nd in his second season, but was sacked with United in 6th after falling out with everyone.
Solskjaer. Came in as a temporary fix but immediately improved things so was given a new contract after about 10 games. Finished 3rd and then 2nd in his two full seasons in charge, but was sacked with United in 7th after being forced to sign and play Ronaldo.
Ragnick. Took over as a "proper coach" who was going to sort them all out, and he got them back in the race for top four for a bit, but then he lost 6 of the last 12 games and they collapsed all the way to 6th again.
Ten Hag. Was also brought in as a "proper manager" and got them playing for a bit. Finished 3rd, won the League Cup. Turned up in the FA Cup but their form went off a cliff in the league again and they collapsed to 8th - United's lowest league finish for a generation. Sacked with United in the bottom half.
See no reason why we won't be back here in three years wondering how Amorim went from finishing 4th in his first full season to being sacked with United in 9th. The players like showing off for a new manager but when the going gets tough they just down tools and wait for the next guy to show up.