"That doesn't look good," Phillies interim manager Pete Mackanin said before Tuesday night's game against the Nationals.
Mackanin said Howard is day to day, but he acknowledged that day to day "could be eight days from now."
Darin Ruf is expected to play regularly at first base while Howard is sidelined.
Other health updates:
• Third baseman Maikel Franco could take live batting practice Wednesday. If that goes well, he could head to Clearwater, Fla., Wednesday night or Thursday morning to face live pitching. If Franco, who has been sidelined since Aug. 12 with a broken left wrist, has no setbacks, he could rejoin the team next week in Miami.
"If he's 100 percent, he'll be back," Mackanin said.
• Outfielder Aaron Altherr has a bruised right elbow. He is available to pinch-run. Nationals closer Jonathan Papelbon hit Altherr with a pitch Monday.
"There's a little bit of a mobility issue," Mackanin said about Altherr's elbow. "There's no structural damage. He's stiff and sore."
Todd Zolecki is a reporter for MLB.com. Read his Phillies blog The Zo Zone, follow him on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.