Kike Salas and
Jon Martin would be my picks
Salas is a better balance of defending and ball playing. But hes 26 and starts for Sevilla. More ready made for instant as perhaps the best CB in la liga - of the regularly used CBs whoscored has him highest, but fotmob has him 4th after Barcelona's Cubarsi, Garcia and Huijsen at Madrid.
Sevilla and Levante have joint best defensive record in the league. I couldnt see a video for him, hes not hyped up like that. He's 6'1 and good at defending the box, very good defending in the air and plenty of tackles and interceptions. Perhaps slightly overly aggressive. But better that hes aggressive than not. He's started the most matches at Sevilla of their CBs with 19 starts in the league although he has had several injuries this season they were all short missing only 3 matches through those injuries, he had 25 starts last season so thats a consideration. Sevilla chop and change a lot and have 5 CBs who all played more than 10 matches. So would he be up for starting all our premier league matches? He also played 1 game at leftback so thats an extra bit of versatility when needed.
Passing is more ordinary but good in some areas. The bottom passing accuracy stats are adjusted for team possession and some other factors. These are very low. %s are low for these higher difficulty passes so he needs to pick and execute his progressive passes better as they have low completion rate and thats an area he needs to do better for a better club than Sevilla.
Jon Martin is better at ball playing. He's only 20 and starts for Sociedad. His defensive stats could be better but hes young and Sociedad have the joint 4th best defensive record in la liga this season. So while he has some rougher areas defensively he hasnt cost his team despite being 20 and a very good ball playing CB. He would be my pick as successor to our Lisandro Martinez and for when Lisandro is out with injury. He's played 22 games and has 7th most mins for Sevilla this season which is a good amount for a young player. Whoscored has him at 14th best CB, Fotmob has him as 12th best CB in la liga this season
He brings the ball out of defence well and is a good passer. Also good in the air, also 6'1 like Salas but 2 cm shorter.
And heres his defending stats which need a bit more work overall, but still some strong points for a 20 year old
Obviously we cant directly compare la liga and the premier league. We dont know that a player in la liga will xcopy their performances to the premier league. But in la liga Jon Martin is performing better overall than Heaven or Yoro are at United in the premier league this season, thus he would be worth picking up despite still being a project himself with the hope he'd be just as good in the premier league.
Mark Pubill is another one, from Atletico but he only played 11 games this season so not a main starter. But Atletico would probably ask more than the above player costs for him as at 22 he's their future, and statistically in those 11 matches whoscored has him as the best CB in la liga and fotmob would have him 7th but with only 11 games hes omitted from the list. So small sample size, high success, but at a bigger club than the others and would likely cost a lot.