Everyone is going in on Martinez now and it's fair game but I'm not sure I would agree with the bolded. His main issues right now as you've highlighted are his lack of incision with his passing in that LCB position, tackling, management of open space and aerials on set pieces. In the middle just like his days in a back four, he's shown more than good enough qualities imo. The issue is whether it's good enough Amorim and whether he'll get a chance there.
In open play, Martinez is smart at influencing aerials so that the opponent doesn't do much/can't direct it well. When the play is in front of him, his anticipation and aggressive tackling can work because he'll be in the central areas of the pitch i.e the two wide CBs will be mostly dealing with the first defensive actions in the channels. He's a good box defender (despite recent showings) and covers well just outside of the penalty area;
his inconsistency is mostly in the wider areas. The only consistent issue I have with him in that CCB role is aerials on set pieces.
It's gamble but one worth trying imo because de Ligt or Maguire aren't exactly perfect themselves.
Mazraoui, Martinez and Shaw?
Super small but not like we're winning any aerials from set pieces right now anyways but that's a pretty good back three on the ball. I'd have Ugarte and Casemiro in front of them with Dalot as RWB. The latter two have good height and jumping power so along with Hojlund (if Amorim tells him, he can actually jump upwards and not just from the same spot) would help mitigate some of the aerial situations.
I don't know about "now" as the issues and concern about him has gone from rather muted to mild to unsure to now a serious concern and that has been in a back 4, before the 3-man system and Amorim arrived at the club. It's more been moments in games in the past; moments that sometimes produce the lapses that are costly, but lapses nonetheless. In the 3-man system, what's different is they're not lapses anymore and if the player he is and has been was the one we got when initially purchasing him, there'd be almost unanimous concurrence that we made a bad purchase. His first season, before the injury, is where he is given leeway and has the credit in the back that would buy him the time and perseverance.
Your CCB is
supposed to be your calmest and best reader and organiser. For me, we don't have an optimal CCB at the club unless you can put Evans in a time capsule and take him back to the self he used to be in his 20's and early 30's. Outside of him, it's a sliding scale of most decent fit to worst fit. Martinez is too rash, too clumsy and just not positionally sound enough for the role. We can talk about the benefits, particularly with playing out from there, but first and foremost you want a solid and trustworthy foil to the two proactive flanking L/R CB's.
Height just isn't the factor that it is talked about as being, for me, certainly not in isolation, anyway. It compounds when the positioning and/or athleticism is bad and worsens still if the short guy doesn't have a specialist niche to his defensive skill set i.e. assured in what he's doing 1on1 because of A, B or C. Baresi is only 5' 9" yet is often classed as the best CB of all time (duelling it out with Nesta, Moore and Maldini in particular in terms of the big/renowned names - otherwise the likes of Figueroa, Kohler and so forth enter the fray), but if you ever watch games of his, the intuitive reading of the game and what to do at
all times is like no other. You don't need to use such lofty names to make a point about this as it's par for the course amongst great, short(er) CB's that they just
know what to do and where they are supposed to be at a rate that it is a template for others to follow. Ayala, Thiago Silva, Cannavaro, Cordoba and so on were so much better at reading play than most around them that their height was not on the table for discussion outside of trivia questions in pubs - problem here is all of those players were renowned for freakishly great elements to their physical capability, particularly leaping, which kills height concerns almost immediately. Martinez doesn't have this in his locker, which is why how tall he is continues to be a hot topic, particularly on corners.
If Martinez was better positionally and particularly if he had that salmon-leap in his locker, you'd not hear a peep about how tall he is every two seconds. I should also point out that an ancient Thiago Silva - who was a literal decade and a half off his prime - showed what being undersized and superb looks like in this very league Martinez is referred as being too short for. I guarantee you if he [Martinez] read the game like Thiago Silva, it wouldn't be a conversation. The same goes for any of the lesser names above - you give Martinez their reading of the game and his thread would look very different (even if he isn't the athlete any of them are). Hell, you don't need to even leave the club to show what superior reading of the game in a short CB looks like as Shaw is never spoken about as too small when he plays the exact same position despite being negligibly taller than Martinez himself.
To the bolded. He is extremely consistent with the things he isn't good at. You can set your watch by them, even, which is why it's prudent to target him to get the expected mistakes he'll make when isolated, and that's the problem with the 3-man backline; there are going to be a lot more times when you're on your own and have to duel 1on1. The guy in the middle has an even greater set of responsibilities as that's the person you're relying on to sweep up after others or be proactive and certain enough as the loose man to know exactly what should be done in the corridors of uncertainty that specifically call out their reading of play.
Never mind my whittering on; the proof of the pudding is whether Amorim even looks to try Martinez in the middle of the 3. He hasn't thus far, and that might well be because Shaw isn't fit... or, it might be that he simply doesn't see Martinez as capable of fulfilling the prerequisites for it. I know which one I think it is.