Frias said he has been hampered by lower back tightness for his last three starts. Although tests ruled out structural damage, Frias said doctors warned him he could injure his arm if he continued to pitch, since he could alter his mechanics in trying to ease the discomfort in his back.
The Dodgers hope Frias can return in late July. Whether he returns as a starting pitcher remains to be seen, since the Dodgers are trying to trade for a starter. Frias, 25, is 5-5 with a 4.39 earned-run average this season.
The Dodgers used 12 starting pitchers all of last season. They could use their 11th starting pitcher on Monday.
On Sunday, the Dodgers filled Frias' roster spot by recalling left-handed reliever Daniel Couloumbe from triple-A Oklahoma City.
Also Sunday, with the Dodgers facing a left-handed starter, Mattingly dropped center fielder Joc Pederson to seventh in the batting order. Pederson is batting .236 against right-handers and left-handers this season, but his OPS is .918 against right-handers and .752 against left-handers.
Enrique Hernandez, who often starts at shortstop against left-handers in place of Jimmy Rollins, started Sunday and led off.