The Niners have been contemplating a change at kicker since Cortez missed a 41-yard field-goal attempt in overtime that cost San Francisco a victory at San Diego, which came two weeks after Cortez had missed a 27-yard attempt that would have won the team's game against Oakland in regulation.
"I felt that Jose seemed to be in a bit of a slump," Mariucci said. "He feels that, too. He's working like crazy to correct it. I explained to him today that I'm going to make a change, but I asked him to keep working, keep training, keep practicing and those sort of things.
"Nonetheless, I'm going to have Jeff Chandler handle our kicking duties. We drafted Jeff because we felt he was a capable guy. Because Jose is in a slump, he's going to get his chance."
Cortez has made 18 of 24 field goal attempts this season and is fifth in the NFC in scoring. But after making 13 field goals in a row, he had slumped badly in San Francisco's last three games, and had missed four of his last eight field-goal attempts - non of which hurt the Niners as much as his last miss.
Chandler, the team's fourth-round draft pick out of Florida, has yet to appear in an NFL game. Now his debut will come in a key NFC showdown matching division leaders on a national stage.
While finally making the change, Mariucci made no commitments to Chandler as the team's kicker the rest of the season, or to continuing to keep two kickers on the roster as the team has done all season.
"I do not have any sort of timeline," Mariucci said. "As of today, both kickers are still on this football team, just like we've had. That hasn't changed. I didn't make any promises that it wouldn't change. I just told both guys to do the very best that they can and we will proceed like that, and we're going forward right now with Jeff.
"I'm simply making a switch right now with anticipation it will be the right thing for us. We're going to look no further than that. We're just going to see how it goes. We're hoping and anticipating it will go well. I do expect Jeff to do a good job for us and I would hope that I won't have to have these conversations again next week."