Lilly, who remained in California for the birth of his daughter on Tuesday, said he received the injury from his brother's elbow in a basketball game on Thursday.
"The timing wasn't good," Lilly said.
Lilly was nonetheless thrown right into action on Friday, pitching a bullpen session within a half-hour of reaching the field at Camelback Ranch-Glendale.
"I've been throwing almost every day the last two weeks," Lilly said. "I can see -- it just looks like I can't."
Lilly was excused from the first two days of camp as he and wife Natasha welcomed Nora Grace.
Lilly joined Clayton Kershaw, Aaron Harang and Chad Billingsley as 80 percent of the starting rotation threw bullpen sessions together.
Billingsley continued to work with pitching coach Rick Honeycutt and bullpen coach Ken Howell on mechanical adjustments with his leg kick.