"It has not progressed the way we would like, so we want to find out what's going on," assistant general manager Thad Levine said. "Any time a pitcher has discomfort in the arm, we have to proceed with caution."
Tolleson last pitched on Friday, then was scratched from his next scheduled appearance when he started feeling the tightness. The initial prognosis was that the situation wasn't serious, but now the Rangers are concerned about the lack of progress in response to treatment.
If Tolleson is shut down for an extended period of time, he may have to start the season on the disabled list. Ironically, he was one of the few Rangers pitchers to stay healthy last season and went 3-1 with a 2.76 ERA in 66 games.
"We first have to assess the nature of the injury," Levine said, "but we're not going to put somebody in harm's way on April 6. We are looking at the medium- and long-term. We won't be myopic."
Richardson to have surgery
Outfielder Antoan Richardson has been diagnosed with a herniated disk in his back and will undergo surgery in Dallas on Friday. He is expected to be out at least two months and will start the season on the DL.
Richardson was signed to a Minor League contract this offseason to provide depth in center field because of his outstanding speed. He was added to the 40-man roster earlier in camp.
• Shin-Soo Choo, who has been limited because of tightness in his left triceps, played catch on Wednesday and said he felt good. He is scheduled to be used at designated hitter on Thursday against the Rockies and is hoping to return to right field on Saturday.
• Ross Ohlendorf, who has been sidelined with tightness in his right groin, is scheduled to pitch on Thursday against the Rockies. He will follow Nick Martinez, who is getting the start. Derek Holland will pitch in a Minor League game.
• Matt Harrison, recovering from back surgery, threw 20 pitches off a mound on Wednesday without any issues.
• Reliever Juan Carlos Oviedo, sidelined with biceps tendinitis, is starting to play catch again but still seems unlikely to be a candidate for the Opening Day roster.