Chacin fills in for Jorge De La Rosa, who was placed on the DL Tuesday with a torn tendon band in his left middle finger.
The Rockies still have not named a starter for Saturday at San Francisco, but it is expected that they'll call up right-hander Esmil Rogers from Colorado Springs.
Right-hander Aaron Cook will start Friday's opener of the three-game set.
In Tuesday's official roster moves, the Rockies:
Optioned catcher Chris Iannetta to Triple-A Colorado Springs, and placed De La Rosa and right-handed starter Jason Hammel (strained right groin) on the 15-day disabled list.
Recalled Chacin from Colorado Springs and purchased the contracts of right-handed reliever Juan Rincon and catcher Paul Phillips from Colorado Springs.
Designated right-handed pitcher Alberto Alburquerque for assignment and transferred right-handed pitcher Greg Reynolds (bone chip in his right elbow) from the 15-day DL to the 60-day DL. The moves opened the roster spots for Rincon and Phillips.