That's interesting. Based on his stats, he very rarely miscontrols the ball and is seldom dispossessed. He is also one of the best midfielders in the world in terms of possession and passing, and is in 3rd percentile for errors. So either you're lying or you're watching a different player than Amrabat.
The problem is you're hyperfocusing on one area. The one area that is his strength.
He's a deep midfielder who is average defensively - although fairly strong physically, not that fast as a counterpoint. He can be making mistakes positionally, defensively, giving away a freekick in a poor area etc. Its not just about giving the ball away with a pass that means something is an error.
That is indeed where he's best, he comes deep to get on the ball and is happiest when he can play a 30 yard pass across from one side of the pitch to the other, or play a long ball into the box from deep for players to try and get a shot from. In that one particular area of playing the ball yeah he's not going to mess up too much because thats his comfort zone.
A midfielder needs to do more than 1 thing.
Also when it comes to the "error" stats you find online they tend to list it as a very specific thing, losing the ball leading to a shot or goal and along those lines. That is not what most people mean when they talk about an error. You can make an error without the opponent being able to convert it into a chance or goal. If you get lucky and they mess up does it change that you left the chance open for them to do more than they did? Next time you might not get lucky. Or it might happen in a really important match which means more than an error in a common match.
He is as a player similar to Jorginho in his first season at Chelsea but more mediocre and stronger physically. Or a third as good Carrick without the defending but more physicality.
That doesnt mean he wont play better than McTom is currently playing. But overall its mostly a sidegrade on when McTom is playing well. He's more of a passer and someone wanting to get on the ball but McTom when playing well is a better ball winner and goal threat. If he was equally as good at ball winning as McTom I'd say he's a better fit in the position because we have Casemiro going forward as a goal threat. But he isnt. So you are sacrificing McTom's better ball winning for Amrabat's higher activity with the ball and better passing. We should in theory be better with the ball but if we lose it we're likely even more vulnerable than we already are on counterattacks and opponent attacks.
Anyway he did well as a fill in left back and didnt have much time in the middle before being subbed. So bring on the next game where he gets to start in midfield and have a proper chance to show what he can do. If he can suddenly do a great job ball winning and defending when Casemiro fails to track back or is caught up the pitch on an attack then thats when he will have been a really good player to bring in. So he needs a few games in midfield to show if he has that. He didnt at Fiorentina and they have a midfielder who can do the 30-40 yard passes but also get forward and score from range. So they certainly didnt need him and are sitting 5th with a game in hand over 2 clubs they could pass with the right result. The best Fiorentina run in ages