In 1987, when Mark McGwire hit 49 home runs as a rookie, Kevin Seitzer was also a rookie, and also had quite a season, collecting 207 hits (which led the league), scoring 105 runs, drawing 80 walks, and hitting 33 doubles, 8 triples, and 15 homers. I am a Kansas City Royals fan, and, at the time, we all had visions of Seitzer being better than that Brett fellow who used to play third. This didn't work out; Seitzer was a good player, but his rookie season was his best effort.
Seitzer was a rather small man with narrow shoulders, a right-handed hitter, not a fast runner, not a great arm, and giving no obvious evidence of great strength. All of this was apparent even when he was a rookie, but he overcame it by being a disciplined player who hit the ball squarely. He was a born-again Christian who sometimes irritated his teammates and managers, perhaps for good reason or perhaps just because, when things go wrong, it's easy to blame the Christian. He never played badly; he never really had a bad year. He never hit lower than .265, but he never could meet the expectations of his rookie season.