Brady didn't struggle in his rookie season.
He may not have struggled but he wasn't a big time QB and playmaker he became. He was for the most part a game manager his first few years as a starter.
In the regular season in his first two Super Bowl wins he had 18 and 23 TDs. In his first Super Bowl playoff run I don't think he threw for more than 180 yards in any playoff game. It wasn't until his 5th season that he started having routine 30+ TD seasons.
Ironically, he has never won a Super Bowl in a season he has thrown more than 28 TD passes.