He was selected 21st overall by the Golden State Warriors in the 1999 NBA Draft out of Southwest Texas State University, but his draft rights were immediately traded to the Pacers in exchange for Vonteego Cummings and a future first-round draft pick.
Foster was part of the Pacers' 1999-2000 Eastern Conference championship team, though he played only 19 games that year as he was fourth on the Pacers' depth chart at center behind Rik Smits, Sam Perkins and Zan Tabak. In his second season he played in 71 games, gaining a spot in the rotation as the Pacers rebuilt for the future. He averaged 3.5 points and a decent 5.5 rebounds per game while averaging only 16.2 minutes per contest.
Foster began the 2001-02 season as the starter, and started 48 games that year before Brad Miller was acquired in a trade with the Chicago Bulls. Nevertheless, Foster played in all 82 games for the first time in his career, averaging 5.7 points and 6.8 rebounds per game.
Miller remained the Pacers' starting center throughout the 2002-03 season, and as such Foster was relegated back to the bench, and he produced his lowest point total yet of his career (2.1 ppg) and grabbed only 3.6 rebounds per game.
Before the 2003-04 season, Brad Miller was traded to the Sacramento Kings, and newly-acquired Scot Pollard began as the starting center. But Foster distinguished himself in the Pacers' season opener against the Detroit Pistons and his play, combined with Pollard's poor play, meant a return to the starting line-up for Foster. The energetic, hard-working center became an important rebounding presence for the Pacers, and his offensive rebounds in particular, combined with well-timed tip-ins, finally endeared himself to Indiana fans.
Thus far in 2004-05 he has had his finest professional season, averaging 9.3 points and 10.1 rebounds per game. Foster became the "go to" man during the Pacers' post-season run which was ended by the Pistons in the quarterfinals.