SURPRISE, Ariz. -- Rangers designated hitter Prince Fielder has been sent back to Texas to undergo a sleep study. Fielder has had difficulty getting a good night's sleep here in Arizona, and the Rangers suggested the study to see if that might help.

"I think he's been feeling symptomatic for awhile," Rangers assistant general manager Thad Levine said. "Since it's only [early] March, we decided to fly him back to Texas to see if we can get him some help."

Fielder is expected to miss three days and be back by the weekend.

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Second baseman Rougned Odor is also out of the lineup with a sore right oblique muscle. He was scratched from Tuesday's game against the Athletics and is listed as day to day.

An oblique injury is something that can linger, but the Rangers are hoping they have caught it early enough.

"We asked a lot of follow-up questions with the medical staff, and they feel it's a precautionary day-to-day [situation]," Levine said.

Pitcher Shawn Tolleson, who has yet to pitch in a Cactus League game because of stiffness in his lower back, said he is making good progress. Tolleson is scheduled to throw in the bullpen on Thursday, and then live batting practice would be the next step.

"I've gotten significantly better every day," Tolleson said. "It feels good when I throw. They are letting me dictate based on how I am feeling. I definitely don't feel rushed at all."

Tolleson has been the most durable Rangers pitcher the past two seasons, with a combined 137 appearances that were the ninth most in the American League over that period. However, he has had minor arm ailments the past two springs that put him in jeopardy of not making the team. There is no such predicament this spring for the Rangers' closer.

Worth noting

• Josh Hamilton, who is dealing with inflammation in his left knee, is off crutches, but he is limited to pool work and has not begun baseball activities.

• Jeremy Guthrie came through his two scoreless innings Tuesday with no issues. He had been dealing with some soreness in his lower back, but he is now at full strength.

• Nick Martinez will start the Cactus League game against the White Sox on Thursday, with Cole Hamels expected to pitch in a B game.