On July 23, 2002 Diggins was traded by the Dodgers along with Shane Nance to the Milwaukee Brewers for a player to be named later and Tyler Houston. The player to be named would eventually be Brian Mallette. He finished the season with the Double-A Huntsville Stars of the Southern League where he went 2-1 with a 1.91 ERA in seven games, all starts. Between the two organizations, Diggins went a combined 8-11 with a 3.20 ERA and 135 strikeouts in 1512⁄3innings pitched. Diggins was promoted to the Major Leagues and made his debut on September 2, 2002 against the Chicago Cubs. He started the game but only pitched 11⁄3, giving up eight runs, seven earned. In five games, all starts, Diggins went 0-4 with an 8.62 ERA in 24 innings pitched with his best start being against the St. Louis Cardinals where he pitched 7 innings allowing 3 runs and striking out 7 while only walking 2. Diggins had 2 quality starts of his 5 games pitched in the big leagues.
In 2003 Diggins began the season with the Double-A Huntsville Stars and made 8 starts before tearing the UCL ligament in his elbow. He went 3-2 with a 2.36 ERA in eight games, all starts. He returned to the Milwaukee Brewers spring training facility where rehabbing the injury failed resulting in season ending Tommy John surgery.
Diggins spent most of 2004 rehabbing from Tommy John Surgery before appearing in only four games in 2004 with the Rookie-Level Arizona League Brewers. He gave-up no earned runs in six innings pitched.
In 2005 Diggins began the season with the Class-A Advanced Brevard County Manatees of the Florida State League. Spent much of the season struggling with setbacks from Tommy John surgery and loss of command. He went 0-1 with a 5.27 ERA in 12 games, two starts.