Rattay has worked wonders for the Niners as a fill-in for the injured Jeff Garcia, who strained ligaments in his ankle during an OT loss to Arizona on Oct. 26. Rattay is 2-1 in this three starts, with victories over St. Louis and Pittsburgh. He has thrown five TD passes and one interception.
Garcia still can't move around well enough to face the Pack, Erickson said.
"He's disappointed that he's not healthy," Erickson said of Garcia. "These games are critical for us, so we have to give our team the best chance to win."
Since being injured, Garcia has been listed a the team's No. 3 QB, but is expected to be moved up to backup for Sunday's game in Green Bay. Rookie Ken Dorsey would move back to the No. 3 spot.
The Packers faced Garcia in the teams' meeting last year. Garcia was 19-of-34 for 156 yards with a touchdown and interception in Green Bay's 20-14 win at 3Com Park on Dec. 15, 2002.