If we do buy a starter quality CB like Skriniar, what do we do while waiting for Toby/Dier to get injured? Are we going to spend 40m+ for a bench player?
If we drop Dier or Toby, that's gonna cause issues too.
While Rodon is not proven and experinced, we bought him for the first team with the understanding that he has the potential to step and over time be good enough to start. We must either commit or sell him.
Also, as far I'm concerned Sanchez is already past tense.
We will hopefully have a ton of games between PL. EL, FA and League Cup so we could rotate the 3 players throughout those competitions much like we do with guys like Lo Celso, Bale, Doherty/Aurier. I think after Thursday we are going to want a better CB pair than Sanchez-Tanganga in the Europa League and likely don't want Toby-Dier constantly playing weekend-midweek-weekend games all season long.
If it were say Skriniar it wouldn't necessarily be 40 million for a bench player because he would presumedly be taking over for Toby as soon as next season (if he didn't take over for him right now). With Toby's age we are going to need to replace him so I am not sure that having that replacement come in now is worse than waiting until he starts dropping off and then absolutely needing someone.
I also don't think we bought Rodon to be a first team player this year at all and if we did that is a huge risk. He can still develop with bringing a new guy.
Edit:
As some comparables to our rivals Liverpool (when healthy) has VVD-Gomez-Matip, City Laporte-Dias-Ake, Chelsea Zouma-Thiago Silva-Rudiger.
I don't think that Rodon is comparable to any of those 3rd choice options so bringing in a 3rd guy wouldn't really put us in a different spot than other clubs that seem to be able to manage just fine with their guys and minutes.