The Rangers called up right-hander Doug Mathis from Triple-A Oklahoma to take Millwood's roster spot. Mathis is 5-0 with a 3.55 ERA in seven starts for Oklahoma, but will pitch in relief for Texas.
Millwood left Saturday night's game against Oakland in the first inning after throwing 12 pitches to three batters, recording two outs. Millwood was injured on his 11th pitch. After he threw one more, he put his hands on his knees in obvious discomfort before being removed from the game.
After Millwood's exit, five relievers allowed four runs over 8 1/3 innings in Texas' 6-4 victory.
Millwood, the Rangers' No. 1 starter, is 2-3 with a 4.88 ERA in nine starts this season. Last season he had two stints on the disabled list with a strained left hamstring.
The Rangers also outrighted right-hander Kazuo Fukumori, currently with Oklahoma, off their 40-man roster.