Your players aren't worse. They will be the same quality as they were 2 days ago (say if your player was 18Q, he still will be 18Q today), the only thing that changes is the amount of stars they are.

If you go onto the market now you will see that the highest available players to buy (the five star ones) will have a higher Q than the ones you has last season. That's because every time you promote, the best players available to you increase in quality by 5Q.