Howard is not swinging a bat yet, but he is running on the anti-gravity treadmill at about 75 percent of his body weight, and he has lost about 15 to 20 pounds in the process.
"I have no idea when or if he's going to be back this year," Amaro said. "But he's doing really well. If he's 100 percent and we get a chance to have him do baseball activities and prepare, then that's a possibility. But if he's not, then there's no reason to bring him back."
Pettibone, who has been on the disabled list since July 29 with a strained right shoulder, will begin a rehab assignment on Thursday with Double-A Reading. He is scheduled to throw about 55 pitches.
Todd Zolecki is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.