I think he's a good enough fit and the Spurs want to do something to push things forward, to keep Wemby happy. Just having another player out there with real gravity will transform Wemby's opportunities and get him some experience operating with an elite PG. I'd expect a decent increase in their win rate with Fox.

They don't want to pay too much for him but with Fox saying that he is focused on joining SAS they should get a good deal for him.

Spurs are still at least 3 or 4 seasons away from contention, Fox fits into that timeline fine; they can move off him or extend him depending on how he works out. In the meantime he makes the team considerably better and gets Wemby some more Ws to keep him positive.