I can only see him going to Sporting when he no longer has it to perform in top level. The fact that he was PL top 3 goalscorer and PL team of season last season, means there is no way he is going to Sporting soon.
It’s going to be either moving to Bayern or staying at Man Utd. If Bayern not really interested even after they have sold Lewandowski, then he would have to come back to us instead.
My guess is, he will stay with us for another season. Even though there is no CL for him, while we won’t be competing for major domestic trophies anytime soon. Maybe he can still try to win a Europa with us, and focus on WC, for keeping up his motivation.
Next summer though I am sure he will end his contract with us, or we won’t extend his contract, and we are going for someone like Nkunku instead to build for future.
But time is really running out for him as he is approaching 38, I won’t be shocked if he decide to move to Sporting next summer, or moving back to Italy for clubs like Milan or Napoli, or basically any clubs that would offer him a chance to score few goals in CL group stage and challenge for some domestic trophy.