Superstars always color matches and help their team win games in times of difficult games. And when they leave there is always something to miss about them, the fan love, fans switching clubs, and most important the team losing top notch performance. Recently, Marquinhos made a statement that he misses the days of Mbappe in PSG. He's sure that their draw wouldn't have happened in the watch of a no "9" like him. Also PSG fans would feel the absence of Messi and Neymar's brilliant skills. On the other hand, Manchester United also should be missing those moments of CR7.
What is no longer there in your club after a certain key player was transferred?
What is no longer there in your club after a certain key player was transferred?
Sometimes it's all about the management or even the complexity of the team. In the management if they think the team does not have a good performance one of the things they do is shuffle the team that needs a new player so they can try new upon participation in the tournaments, if the result is good they will still as is but if not they will make a shuffle again reason why is also about the money of course they pay a lot of money to the players so if they can find better but with the lower price or a better player with higher price to check if its all worth it.
There are won matches because of these superstars and there's no doubt such moments wouldn't be missed by a fan.
When Kawhi left Toronto, I was very upset. 
Now I see him injured with the Clippers and he will never win a Championship again. If he had stayed with Toronto, I'm sure he would have had another chance, with the team the Raptors had. No other star player in NBA history would leave the team he just won a Championship with. But he chose going home vs competing for another Championship. Do I think it's slightly selfish? Yes. But I guess I can thank him for helping my team win a Championship at the very least. 
No certainty. Injuries, unfortunately, do happen and are already anticipated by players because it could be a part of their job as athletes. At the end of the day they are still employees of Franchises who only follows the management. As fans I guess it is safe to say that we should also adjust with our expectations and still be happy as long as we are seeing these superstars on matches even with a different uniform. Retirement on the other hand, for me hits deeper. Now I see him injured with the Clippers and he will never win a Championship again. If he had stayed with Toronto, I'm sure he would have had another chance, with the team the Raptors had. No other star player in NBA history would leave the team he just won a Championship with. But he chose going home vs competing for another Championship. Do I think it's slightly selfish? Yes. But I guess I can thank him for helping my team win a Championship at the very least.