Manager Ned Yost said Vargas will throw between 30-40 pitches, with a hard cap at 40. The hope is to gradually extend the lefty by approximately 20 more pitches over a few subsequent simulated sessions and potentially avoid a Minor League rehab start.
"He's definitely feeling good right now or we wouldn't be throwing him tomorrow," Yost said. "He's ready to take that step. It's important to get him built back up again to 80 or 85 pitches to be ready to pitch five or six innings at a time."
Vargas said he just wants to see if most of the pain has subsided and how his arm reacts after so much down time.
"When it was bothering me, it was when I was turning and throwing the ball, I'd feel a discomfort I hadn't felt before," he said. "It's been awhile since I've [thrown] hard off a mound, so you just don't know."
Vargas was 5-2 with a 4.10 ERA in 41 2/3 innings during eight starts before the elbow troubles landed him on the DL on June 13.
• Yordano Ventura (ulnar nerve irritation) was slated to make his second rehab start with Triple-A Omaha on Wednesday night.
Chris Abshire is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.