Even if he lost his speed, he can still play as a regista. As good or better than Pjanic at everything there. Pjanic can compete with Can and Bentancur for RCM. Or might still have enough legs left to be a quality B2B/mezzala himself.
I wouldn't sign him in summer, but with limited options in Jan, if he's the best we can do and if it's financially viable, bring him in.