Locke threw off a mound Saturday morning for the first time in more than a month, showing progress from an injury suffered at the start of spring training.
"I don't know what we'll do next, but for today it was good," Locke told reporters of the session, according to the South Florida Sun Sentinel. "I feel a lot better than I was feeling before, that's for sure."
Mattingly said there is no timetable for Locke's return to the Marlins.
"Just going through it in the past, it'll be a little while," Mattingly said. "Probably all the way through April, late April, May, middle of May. Something like that."
Locke was 9-8 with a 5.44 ERA in 19 starts and 11 relief appearances for the Pittsburgh Pirates last season. He signed a one-year, $3 million contract with the Marlins during the offseason.