When will Brandon Webb pitch again?

The answer remains unclear.

Barring a setback, the return of the Diamondbacks' right-hander in May is "highly likely," according to a major-league source.

Webb, however, has yet to make a significant breakthrough in his recovery from shoulder surgery last August, and the Diamondbacks seem to be bracing for an extended absence.

"I have no idea how to establish a timeline," Diamondbacks general manager Josh Byrnes said Thursday morning. "We have to prepare to play with that unknown."

Webb, who underwent shoulder surgery last August, received a "very encouraging" report Wednesday in his meeting with team doctor Michael Lee, according to the source.

Lee found no issues in Webb's shoulder and reported normal progress, the source said. Webb, though, has not thrown off a mound since March 4 and has yet to face hitters this spring.

The Diamondbacks' season-opening rotation will consist of right-handers Dan Haren, Edwin Jackson, Ian Kennedy, Rodrigo Lopez and Kris Benson.

The team has been trying to trade for another starter to slot behind Haren and Jackson.