The 2007 NL Cy Young Award winner has been recovering from a strained right-ankle tendon. He was acquired last month in a trade with the San Diego Padres and is scheduled to make a rehab start Monday for Triple-A Charlotte.
"We cannot rush with him," Williams said on the team's Web site. "We have to be careful about how we're going to use him."
Williams said Peavy threw a rigorous bullpen session Friday and later said he experienced some back tightness.
In two other two rehab starts, Peavy is 0-1 with a 3.86 ERA in seven innings, one walk and 10 strikeouts.
This season, he is 6-6 with a 3.97 ERA in 81 2/3 innings over 13 starts.