Haren made his professional debut with the New Jersey Cardinals in 2001 and was 3-3 with a 3.10 ERA in 12 appearances, with 8 starts.
He also struck out 57 while only walking 8.
He then made 28 starts in A-ball in 2002 for the Peoria Chiefs and Potomac Cannons, finishing with a combined ERA of 2.74.
He started 2003 in Double-A with the Tennessee Smokies, but was promoted after eight starts to the Triple-A Memphis Redbirds.
He was a combined 8-1 with a 2.68 ERA in 16 starts in 2003, with 84 strikeouts and was selected as the Cardinals minor league pitcher of the year.

Haren made his major league debut at the age of 22, on June 30, 2003.
He was the starting pitcher against the San Francisco Giants and allowed four runs (two earned) in six innings to pick up the loss.
He recorded his first Major League win with a six-inning start where he allowed only one run against the Los Angeles Dodgers on July 19.
After finishing 2003 with a 5.08 ERA for the Cardinals in 14 starts, he was sent back down to AAA Memphis for the 2004 season.
He received a late-season call-up, and made five appearances in the postseason for the Cardinals, including two in the World Series.
The Cardinals were swept by the Boston Red Sox, but Haren pitched well, tossing 4​²⁄₃ scoreless innings.
Haren made his professional debut with the New Jersey Cardinals in 2001 and was 3-3 with a 3.10 ERA in 12 appearances, with 8 starts. He also struck out 57 while only walking 8. He then made 28 starts in A-ball in 2002 for the Peoria Chiefs and Potomac Cannons, finishing with a combined ERA of 2.74. He started 2003 in Double-A with the Tennessee Smokies, but was promoted after eight starts to the Triple-A Memphis Redbirds. He was a combined 8-1 with a 2.68 ERA in 16 starts in 2003, with 84 strikeouts and was selected as the Cardinals minor league pitcher of the year.
Haren made his major league debut at the age of 22, on June 30, 2003. He was the starting pitcher against the San Francisco Giants and allowed four runs (two earned) in six innings to pick up the loss. He recorded his first Major League win with a six-inning start where he allowed only one run against the Los Angeles Dodgers on July 19. After finishing 2003 with a 5.08 ERA for the Cardinals in 14 starts, he was sent back down to AAA Memphis for the 2004 season. He received a late-season call-up, and made five appearances in the postseason for the Cardinals, including two in the World Series. The Cardinals were swept by the Boston Red Sox, but Haren pitched well, tossing 4​²⁄₃ scoreless innings.
Dan Haren