Fontenot began the season as the starting second baseman, but when Ramirez dislocated his left shoulder on May 8, the Cubs needed a replacement. They began the season without a true backup at third. Ramirez isn't expected back until the first week of July, and losing Aaron Miles to a shoulder injury and Ryan Freel to a hamstring injury didn't help matters.
On Friday, Bobby Scales started at third and Fontenot was at second, but they flip-flopped on Saturday.
"I think that's what you're going to see," manager Lou Piniella said of Fontenot at third. "At the same time, we'll continue to work [Jake] Fox at third base just in case we need a double-switch, just in case there's an injury. We have to be prepared."
Fox is a hitter without a position who the Cubs want to take advantage of. He was called up Wednesday, and at the time, he was leading the Pacific Coast League in batting average (.423), home runs (17) and RBIs (50) -- what Piniella called "scary numbers."
Fox, who was a catcher at one point in the Cubs' system, has been taking grounders at third and working with bench coach Alan Trammell.
"We're doing this to give us a little extra option," Piniella said. "It's asking a lot of a young man who hasn't played there -- he played first base in Triple-A. We only have one infielder, so if there's an injury and we have to make a double-switch, at least it gives us another option.
"I've got Derrek [Lee] at first and I've got [Micah] Hoffpauir who plays there, too. Unless I can use three first basemen, [I can't start Fox]."
When the Cubs face a right-handed starter, expect Andres Blanco at second base, not Scales. Blanco was a switch-hitter, but now bats only left-handed. The Cubs didn't know that until Blanco was called up Wednesday.
Ramirez was hitting .364 with four home runs and 16 RBIs before he was hurt. The fill-in third basemen have combined to bat .200 with four home runs and 16 RBIs. So far, Fontenot was batting .254 when he plays second, and .200 when he plays third.
"[Fontenot] is basically our best third baseman, our best option over there," Piniella said. "We feel very comfortable with him over there. I hope he feels as comfortable as we feel with him."
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.