This in itself is a good lesson, but nothing will be taken from it.
Football fans are generally reactive morons, swinging from one thing to another.
What could also be the case is that player A plays OK at club A l, but then plays much better at club B where the environment is superior for growth, improving his standing. An endless football story.
I've said before but I'm not really concerned that we get rid of players who go elsewhere and do well, and surmise that this player would have done exactly the same here with a bit more patience or greater role. They might, they might not. The key for us for years has been bringing together players and coaching who as a group fit in and do well in our environment and improve this club. The shit we often buy will continue to be the problem rather than the grass is greener of who we get rid of. When this happens over and over again it's not one or even a few decisions, it's endemic amateurism.