He has exceeded the reasonable expectations. Top 4 finish (possibly 3rd place), a trophy, and an FA cup final--we're quite unlikely to win it, but even so, that's not nothing. And we may yet win it, you never know. What is it they say about derbies?
It did start to come apart in the last third of the season, but we've also had some serious injury woes as well as losing Casemiro to two consecutive red cards that were both highly questionable. Then a season-ending injury to the team's best player, and a range of other minor injuries as well. On top of that, we've played very close to the maximum possible number of games that a team can play in a season, and that's after a winter World Cup where much of the squad played quite a few games. These things are relevant. They're the reason things got tough towards the end.
When taking all of that into account, he has gone above and beyond. His signings have worked well, by and large. Our season's bottom line has come out ahead of rational expectations, even if we do lose the FA Cup final. At the start of this season, if any fan had been asked if they'd take CL qualification and a cup win for ETH's first go at the job, anybody with any sense would have screamed 'yes!'
There have been some fair questions about his team selections, especially when it comes to Sancho and Weghorst; but all in all, he has done as much as any sane person could possibly require, and he has more than proven that he deserves a serious chance to keep this going, even if next season proves tough with CL football and the desperate need for more signings than it would be fair to demand in one summer.
Throughout the first half of this season, everyone was gushing with enthusiasm and praising him for turning United into a tough team that won the games we should be winning (aside from those first two games when he was clearly just getting his bearings). For some time, there was even talk of a potential title challenge. Then fatigue set in and it was clear that the absurd number of games was taking its toll on a squad that wasn't really used to it, but that'll get better. One of our key players, Bruno, has played more football this season than literally anyone else in Europe. That sort of thing matters. That'll have an inevitable effect as a season wears on, and it would be unintelligent to ignore that and attribute all problems to the manager's own work.
So I'm very optimistic for the future. There are good reasons to be, and the concerns people have had and unfairly attributed to ETH were clearly the product of the aforementioned difficulties. Injuries, fatigue, lacking a dependable striker because none were really available on the market, these aren't to be seen as failures of ETH's management. The fact that we did quite well in the end despite these factors says all that needs to be said.