Billingsley has a partially torn ligament and is trying to avoid Tommy John surgery by receiving a pair of platelet-rich plasma injections.
He has played light-toss catch for a week and now will ramp up to see if the ligament can withstand the rigors of pitching. If not, he will probably undergo surgery that will sideline him for the entire 2013 season.
"He'll have to let it go," Mattingly said.
In other medical news, Mark Ellis said he felt much better after missing Wednesday's game with the flu, but Mattingly decided to give him another day off and start in his place Nick Punto, who had three hits and scored four runs Wednesday.