I'd say he's the long term replacement for Xabi. Xabi is 29 and has 2 years left on his contract. If Sahin can do what he does well enough for the next season, they can think seriously about letting him go rather than giving him what would likely be a rather expensive extension. I wager that if Sahin plays well enough, Xabi might become available end of this season. It's a big if though, and even if he does, I suspect he might want to wind down his career at Sociedad.