What teams can prepare for a new season without buying players is two folds. They could train better, see the videos from last season and see what mistakes were made and try to cover their weaknesses and then train accordingly to be better at what went wrong last year.

And secondly, they could also try to figure out a way to get "fatigue" injuries lower, which means they need to make sure that the players are not tired all the time, they can check their schedule, they can check their team squad and depth, and they can try to arrange a way for players to replace each other, even out of position changes could happen if you could give the players enough time to train for it. Obviously every team also gets new players and even sell some players, but these other two things are very important too.