I'm gonna go out on a limb here and say I don't think the two are directly linked - we thinned the squad down massively over the last year, and we already have both the squad-space and the money free to add Tielemans without needing something else to shift first, so I don't see Xhaka's situation impeding that move.
As for Xhaka, it's kind of up to him whether he wants to stay or move on. Tielemans coming in would most likely mean he'll spending a lot more time on the bench, but then again I think the 5-subs thing is going to be a pretty big deal next season, and I can really see it changing how teams think. I have a feeling we'll see a move towards what they've been doing in rugby where you have players who start games and players who finish games, and a lot more switching between systems if something isn't working - Xhaka gives us the option of a double-pivot when we need it, which might be useful? Don't know how many minutes that will translate to, but it's a consideration.
If he does go, it's 50/50 whether we replace him - whoever comes in would be looking at the same situation, so it would depend on the player and the price.