Defending is the easiest thing to do in football, never forget that.
Which is why the sport is a low scoring game.
Defending good is the toughest thing in football because you need whole team to work as a unit.
Yeah but look at how many are failing to make an impact too.
Team leading? Sub off attacking player.
Team losing? Sub off underperforming attacking player.
Easier to get continuity at CB. But of course differs from case to case and always the risk of ending up longer periods in the sidelines as CB. But if a team wants him they'll probably have a plan for him.
It's easier to sub atacking players as you don't risk your defensive stability. That's why you rarely see coaches taking off CBs in close games unless they're injured, on the verge of a second yellow or having a really bad day. But you don't see often unproven CBs coming on "to get experience" for the last few minutes in 1-0 type games like it happens with forwards.
Another reason for subbing attacking player is they usually ran more than CBs (who in general cover the least ground) so they get tired faster. And if you have a young fast skillfull winger to bring on for the last 10 minutes against tired opponents ofc you'd rather do that than bring on a defender.
Getting continuity at CB might be easier once you prove yourselves but getting established in the XI is harder. Especially if you have two experienced CBs who have worked well together for years like Garcia and Aridane in Osasuna. Breaking that pair won't be easy for a 19 yo kid who has never played above the Segunda B/Primera RFEF level. But ofc if one or both of them leave there would be a chance for Chadi to come in and impress coaches from the start.
That's also a reason why we should loan out players (those we don't want to give up permanently yet) early so they can start preseason with their new teams and improve their chances to succeed.
Should be the case with Chadi (who I just don't see a spot for in the first team next season) and Pablo Torre as well. Xavi has seen him training for a whole season and he doesn't believe he's good enough for more minutes that he got so far so almost impossible a preseason with first team would change his mind. Send Pablo on loan to Villarreal, Betis, Celta or whoever is interested and let see if he can prove himself there.
Another thing is that clubs who want our players on loan in May/June are usually genuinely interested in them and those players are one of their primary targets. Much different than loaning them in late August to clubs who for some reason couldn't get their 1st, 2nd, 3rd and potentialy 4th or 5th prefered option so in the end they settle for one of our loanees.