In 2005, Sillman played his first season of Class-A ball at Quad Cities and racked up an impressive 8-2 record while recording a team-high 22 saves (3rd in the Midwest League) and a 2.74 ERA throughout his 56 games and participated in the 2005 Midwest League All-Star Game.
Sillman spent this season playing High-A ball for the Palm Beach Cardinals where he again led the team and was second in all of Minor League Baseball with 35 saves in 57 games. Sillman also led his team with a 1.10 ERA and recorded 13.50 strikeouts per nine innings pitched, ranking him second in Class-A.
Sillman had a strike out-to-walk ratio of better than 4 to 1, striking out 86 while walking just 20. His home ERA was a microscopic 0.56 in 30 games. He was the only member of the Palm Beach Cardinals to be named to the Florida State League Mid-Season and Post-Season All-Star team.