You're making my argument for me. Pep rotates when he absolute has to. He rotated practically the whole squad against Leverkusen and they played like shit. He's got 4 attacking mids he likes for 3 spots Doku, Reijnders, Cherki and Foden. I don't see Semenyo breaking into that rotation. That leaves coming on as a sub. Is he better than Savinho? Of course. But would I pay £65 million for a sub? Fuck no. Plus there's Bobb and Marmoush as well. Are they better than Semenyo? No. But adding him just adds to the logjam of players Pep doesn't trust.
And Rodri doesn't seem like he's coming back very soon. Has he even been available? Kovacic is out til at least February. You've got Nico and Bernardo in defensive midfield, that's it. If they need a sub/rotation player somewhere, its defensive midfield.