Millwood threw just 12 pitches and faced three batters on Saturday before leaving the game with two outs in the first inning. He started the game by getting Kurt Suzuki and Daric Barton to fly out, then came up hurting on his third pitch to Jack Cust.
"I felt it on the 1-0 pitch to Cust," Millwood said after the game. "I tried to throw another pitch and I felt the same thing. It was a bit of a stabbing pain, even though I hadn't been stabbed."
On the season, Millwood is 2-3 with a 4.88 ERA in nine starts. He is eligible to be activated on May 26.
Because of the 8 1/3 innings the bullpen had to throw on Saturday, the Rangers opted to go with Mathis, who has pitched well for Oklahoma so far this season. In his last start on Tuesday, Mathis allowed two runs, both earned, on eight hits with six strikeouts over eight innings. On the season, Mathis is 5-0 with a 3.55 ERA over 45 2/3 innings in seven starts.
"Right now, I'm putting the ball in the zone and making pitches," said Mathis, who relies mostly on his sinking fastball.
Mathis, who was scheduled to start Sunday for Oklahoma, will initally serve as a long reliever for the Rangers.
"I was pretty surprised last night," Mathis said. "I was just getting home from the game against Fresno when I got the call. My parents are actually flying in from Arizona [Sunday] for the game, so that's nice."
Manager Ron Washington said Mathis might be the starter on May 20, when the team is at Minnesota.
Mathis was added to the Rangers Spring Training roster in March, posting a 4.15 ERA in five relief appearances in the Cactus League. He was selected in the 13th round of the 2005 First-Year Player Draft out of the University of Missouri.
The Rangers also outrighted right-hander Kazuo Fukumori, who is currently at Triple-A Oklahoma, off the 40-man roster on Sunday. The team now has 39 players on the 40-man roster.
Drew Davison is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.