We had a guy who was 15-25 end up winning 107 games in GW1. Some of his guys had slow starts. You'll see this a lot in the game.
However, beware, some years, 19 of your 25 guys underperform their ratings for the entire season.
Btw, if you don't like your team, you can join one of the five openings in Glavine 500. There are immediate contending teams available, rebuild teams, or tweener teams available in the league.