Webb was scheduled to pitch for Triple-A Round Rock on Friday night but instead will be evaluated by team doctors for the Rangers.
Webb is trying to come back from right rotator cuff surgery. The 2006 NL Cy Young Award winner hasn't pitched in the majors since the 2009 season opener for Arizona.
Before getting hurt, Webb had pitched more than 200 innings in five consecutive seasons for the Diamondbacks.
The Rangers knew when they gave Webb a $3 million, one-year deal last winter that his availability to pitch in the majors was uncertain. They were hopeful that he might join the rotation by midseason.