Really interested to see how certain players fit into Amorim's system if he does indeed implement some variation of a back three in his first game.
Onana obviously plays as the first choice GK.
Central Defender
I think the optimal candidate to play as the central option is De Ligt, with Maguire as second choice for now and anyone else would be after that (bar Maybe Yoro). Evans might play there very occasionally unless we have another injury crisis, but I think his expected position might be a squad option at LCB more than CB. I'm not quite sure on the pace of De Ligt and Maguire fitting into Amorim's 343, but I think they'd be less exposed here than trying to be shoehorned in elsewhere.
Right Central Defender
I think Lindelof will start here but once Yoro is fully fit I can see him nailing down this position as well as a younger player with pace and some support around him. I also would not be surprised to see Mazraoui here as time goes on, if we have Shaw/Malacia fit for longer periods or if someone else plays the Wing Back position.
Left Central Defender
Martinez is the obvious candidate here, then probably Shaw or Evans as the back-ups. Seems more clear-cut than the others.
Right Wing Back
Dalot is probably going to get this one, but may be asked to play on the left sometimes until the squad is more settled with the system and injuries.
Left Wing Back
I actually have a sneaking feeling that Amorim may try to re-purpose Antony in this position. I think Antony is out the door at the earliest opportunity for what it's worth, but this is his only chance to play IMO. He is already third choice Right Winger as it stands and if we are potentially playing one less attacking player, then Bruno, Mount, and maybe even Eriksen become further competition in the position he would play as part of a front three - so, he could actually be 7th or 8th choice unless he's willing to have a go at the LWB role. I think he might actually be OK there, he does run to be fair to him. If he doesn't have to think too much, just overlap and first-time cross, get back in position etc. It could work. If not, look at loaning him out in January and selling in the Summer. Shaw/Malacia will obviously play here and maybe he will try and trust Rashford/Garnacho? But it just seems unlikely that will work to me.
Central Midfielders
Ugarte and Mainoo is probably expected to be the main settled pair, with Casemiro and Eriksen as the back-ups. However, I think we'd be aiming to let these two go in the Summer so need a bit more. If we're in form and expected to dominate some games at home, maybe he plays Bruno here to get another attacking player involved, but I'm not sure this would ever be a permanent thing.
No.10's / Wide Forwards
This is where it gets a bit difficult, as Bruno and Mount work when we are playing more a bit more narrow and want to pack out the midfield. I think Amad is more suited to this than Rashford and Garnacho. But it'll be interesting to see if Amorim favours more of two wide forwards and a central striker, or a no.10 and two strikers/wider forwards. This will be flexible IMO.
Striker
The hope is on Hojlund, but I wouldn't be surprised if Rashford ends up playing as much if not more as a lone striker. Zirkzee is more likely to be a back-up for me. I'm really just not sold on him fulfilling a particular role well enough. I'm probably writing him off too soon, but he feels like a squad player to me. His best bits of play have been taking the ball deeper and finding some nice wide passes but he lacks the pace to then get in the box. He's such a strange player, his physical attributes don't suit his playstyle. But yeah, for me he is a third choice striker under Amorim with the capability to maybe be a back-up no.10 as well (but even there he's behind a fair few players).
Onana
Lindelof De Ligt Martinez
Dalot Casemiro Ugarte Mazraoui
Bruno
Garnacho Rashford
Tough to know what he'll do first game, he might not even go with the 343 like everyone expects. Amad is playing well at the moment, but you'd think he won't leave out Bruno. Hojlund also must be a huge consideration if he's going with one striker, so it's very hard to find the right decision out of Amad, Garnacho, Rashford, and Hojlund to play with Bruno as the three attackers. It's only really 'easy' to leave out Mount, Zirkzee and Antony. But that's Amorim's first problem to solve, one of many he'll have to do in this job. Good luck to him!
Should be winning this game, but of course they've just managed their first win back in the PL so will be at the most confident they've been when they play us, along with well rested!