Willis, a former National League Rookie of the Year (2003) and runner-up for the Cy Young Award (2005), has won just three times in the majors since completing a 10-victory season with the Florida Marlins in 2007.
He was scheduled to start Sunday against Milwaukee, but was replaced by Matt Maloney after suffering back spasms.
Willis was 2-8 in 24 appearances over two-plus seasons with Detroit, then 1-1 in six appearances with Arizona in 2010 before joining the Reds to begin 2011.
The Oakland, Calif. native had pitched to a 4.21 earned run average through his initial 11 starts with the Reds through Sept. 5, but was ripped for eight runs on nine hits in 3 1/3 innings against the Chicago Cubs in his last start on Sept. 12.
Willis got a no-decision at Houston on Aug. 3 - a 5-4 Astros win - after giving up two runs on eight hits in six innings.
He's 3-2 lifetime against Houston with a 2.65 ERA.
The Astros counter with lefty J.A. Happ, who's won two decisions in three unbeaten starts since ending a four-start skid.
The 28-year-old was a 5-2 winner over his former team - the Philadelphia Phillies - in his most recent start on Sept. 13 after allowing a run on four hits i six innings.
Previously, in two starts, he allowed four runs on eight hits in 12 innings while getting a win and a no-decision against Pittsburgh.
Happ is 1-1 in three career starts against the Reds with a 7.27 ERA and is just 1-9 in 12 road starts in 2011.
On Sunday in Chicago, Clint Barmes' two-run single in the first proved to be the game-winner as the Astros held on for a 3-2 win over the Cubs in the finale of a three-game set.
J.D. Martinez drove in the other run for the Astros, who had dropped their previous three games. Brett Myers (6-13) won his third straight start as he gave up just two runs -- one earned -- and seven hits with one walk and five strikeouts in 7 2/3 innings.
Maloney (0-3) was roughed up in his spot start, as he was tagged for seven runs -- six earned -- on nine hits in just 1 2/3 innings.
Cincinnati is 7-5 versus the Astros this season.