Mauer, who did not play Saturday, was not in the lineup for Sunday's game against the Cleveland Indians. The Twins recalled catcher Wilson Ramos from Triple-A Rochester, and he'll make his major league debut Sunday.
"Joe is really sore," manager Ron Gardenhire said before Sunday's game. "We're going to rest him until it is right ... We don't want to risk things. If I play him, it will never heal. I have to protect Joe. He said it really hurt just to put his cleats on."
Mauer hurt the heel Friday night when he awkwardly stepped on first base as he lunged to beat out an infield single. Rookie Drew Butera played all 11 innings Saturday in a 5-4 loss to the Indians.
"I was really nervous the whole game with only one catcher," Gardenhire said. "When there was that collision, my blood pressure went crazy."
In the 10th inning, Cleveland's Lou Marson crashed into Butera trying to score.
Gardenhire said he wanted to try and get Ramos to Cleveland before Saturday's game and when he couldn't, the manager went around the clubhouse asking for volunteers to serve as an emergency backup.
"Orlando Hudson said he'd do it," Gardenhire said of the chatty veteran who has never played any position but second base in nine seasons. "He'd wear out all the opposing hitters just talking to them."
With closer Joe Nathan out for the season after elbow surgery, Gardenhire is particularly wary of losing another key player.
"I really don't know what to expect," Mauer said of the injury on Saturday, according to The (Minneapolis) Star Tribune. "I've never had it. I'm hoping it doesn't last too long, hopefully a couple days, I just don't know."
Mauer, the reigning American League MVP, signed an eight-year, $184 million contract extension with the Twins during spring training.
To make roster room for Ramos, right-hander Pat Neshek was placed on the 15-day disabled list with inflammation of his right middle finger.
Neshek had a 4.15 ERA in 4 1/3 innings over six outings. He missed all of 2009 rehabbing from Tommy John surgery in May 2008.
"He said his finger and hand just didn't feel right, so we'll send him down and let him build up arm strength," Gardenhire said. "He walked a guy and hit another the last time out and told me he needed to pitch."
Information from The Associated Press was used in this report.