Very poor analysis, in my view. Firstly, when fully fit, it's not like he's a defensive liability - on the contrary he has great defensive awareness and rare one on one ability. Secondly, it's not like our system requires 3 CBs who are great in the air and very fast. It does require at least one CB with the sort of passing ability he has. Plus he's one of few players in the squad with the right mentality. If it wasn't for the injuries, he'd be a key player.
Defensive liability would be harsh, it's not like he's dropping several clangers every game. But he has plenty of poor games where his weaknesses exposed. He's not anywhere near the actual good centre backs in the league - Gabriel, Saliba, Konate, VVD - these players just deal with situations with ease. Why? They have the physical attributes to deal with them. They have the height, physicality, pace to deal with the aerials, deal with the pacey attackers.
I'm not sure how we think Martinez has rare 1v1 ability, pacey players can have him on toast.
Did you actually disagree with my sentiment that there's barely a premier league side who would actual want to buy him and have them start for them at centre back?
Again we're constantly resorting to trying to compensate. We're constantly trying to "get away with" players who don't have the necessary physical attributes to consistently perform the way we need them to.
Have a midfielder who can't run? Well just have others do the running for him.
Have a striker who moves at snails pace? Just put some pace around him
Have a centre back who looks like he's running through treacle? That's ok, the other quicker centre back will have to bail him out
Have a centre back who gets bullied in the air? Don't worry, you have a taller centre back next to them so they'll pick up the slack
There is only so much space other players can cover and only so many players they can mark at once. You need your centre back to deal with a ball played into the channel and get to the ball first ahead of the pacey attacker. When the attacker pulls onto your centre back at the back post and the cross comes in, you need your centre back to go dominate that aerial ball. You cannot rely on others to do your job.