According to Nick Piecoro of AZCentral Sports, manager Torey Lovullo revealed Wednesday that Walker will undergo Tommy John surgery. Piecoro noted Matt Koch will start for Walker in Friday's contest against the San Diego Padres.
ESPN.com noted Walker, who was on the disabled list with what was deemed forearm tightness, underwent an MRI on Monday and traveled to New York on Tuesday to seek out another opinion to see if his ulnar collateral ligament was torn.
Zach Buchanan of The Athletic said the injury was a partial UCL tear.
Walker has made three starts this season and sports a 3.46 ERA and 1.54 WHIP. He spent the first four seasons of his career with the Seattle Mariners but appeared to turn the corner in 2017, his first year with Arizona.
The right-hander posted a 3.49 ERA and 146 strikeouts in 157.1 innings in 2017 after finishing with an ERA above 4.20 in his last two seasons with the Mariners.
The Diamondbacks still have Zack Greinke and Robbie Ray to anchor the rotation, but they will be hard pressed to replicate last year's playoff appearance if others such as Koch or Braden Shipley don't effectively fill the void for Walker.