Complex sentence: Nieuwendyk's first season in the NHL was a short one, only playing nine regular-season games, which, because he played fewer than 25 games, enabled (1987--88) to be considered his rookie year.
Split into simpler sentences: Nieuwendyk's first season in the NHL was a short one, only playing nine regular-season games. Because he played fewer than 25 games, his second season (1987--88) in the NHL was considered his rookie year.