Soto had been sidelined since mid-May after undergoing arthroscopic surgery to repair a torn meniscus in his right knee.
"I had a good week [at Iowa] and was in plenty of games," said Soto, who was 3-for-16 (.188) in five games with two doubles.
Soto was batting .161 when he was sidelined with the knee injury, which he felt May 16.
"I had a lot of at-bats [in Iowa] and tried to find myself and I think we're on the right track," Soto said.
The Cubs had talked about keeping three catchers, but decided Castillo needs regular at-bats to develop. Steve Clevenger is the backup catcher, although he started at first base on Monday.
Soto has been eager to return.
"After the surgery, all I wanted to do was come back ready as fast as I could," Soto said. "It was a little frustrating -- being on the DL is no fun."