Soto, the 2008 NL rookie of the year, was injured while taking batting practice Wednesday. He could be out for a month. The move Saturday is retroactive to July 7.
The Cubs had hoped to claim a veteran catcher off waivers but were unable to do so, manager Lou Piniella said. And they decided not to bring up a catcher from Iowa, even though they have a doubleheader Sunday.
"It's a difficult position to find on the streets," general manager Jim Hendry said. "Right now we'll go with what we got and see what happens, get to the [All-Star] break and reevaluate."
Koyie Hill, Soto's backup, will be the main catcher and utility player Jake Fox is now the backup. Fox, who can also play third, first and the outfield, said he caught two games earlier this season at Iowa before being called up.
"We're not scared of using Fox," Piniella said Saturday.
Soto is batting .230 with eight home runs and 27 RBIs in 69 games this season.
Fuld, who was optioned to Iowa from Chicago last Monday, has batted .364 in six games for the Cubs this season.