In the opener, Corey Luebke tossed seven brilliant innings to lead the Padres to an 8-2 thrashing of the Rockies.
Luebke (6-9) took a no-hitter into the sixth inning before having it broken up on a two-run home run by Mark Ellis. That turned out to be Colorado's only hit. Luebke struck out nine and walked three. Ernesto Frieri pitched the final two innings.
Kevin Millwood (3-3) was roughed up in 3 1/3 innings, giving up seven runs -- six earned -- on nine hits for the Rockies.
Chacin, a 23-year-old veteran of 51 major-league starts, has dropped two straight since defeating the Los Angeles Dodgers for his 11th win of the season on Aug. 28.
He allowed four runs on six hits in five innings of that 7-6 victory, but has given up 10 runs in 11 innings since while losing to Cincinnati and San Francisco.
An undrafted free-agent signee in 2004, Chacin was 0-1 in nine appearance as a rookie in 2009 and won nine game in 28 appearances in 2010 before reaching double digits this season.
He's 1-0 in five career meetings with the Padres.
San Diego counters with right Mat Latos, who won a career-best 14 games last season, but has struggled in 2011.
The 23-year-old has won just twice in his last 10 starts, defeating Pittsburgh on Aug. 7 and the Rockies on Sept. 4 in San Diego.
He was a tough-luck 3-1 loser to the Giants in his most recent outing on Sept. 14 after allowing three runs on seven hits in seven innings.
In spite of the poor record, Latos' earned run average has actually dropped - from 4.10 to 3.73 - since late July.
He allowed two runs in seven innings in the aforementioned defeat of Colorado and is 2-2 lifetime against the Rockies in five starts.
The Rockies have won nine of 16 against the Padres this season.