Heap has a pulled right hamstring, and Zbikowski has a back injury.
Zbikowski is awaiting the results of an MRI exam to determine the severity of his back injury. He could end up on injured reserve if he can't come back within the next two weeks.
"We'll just have to see," Ravens coach John Harbaugh said in regard to Zbikowski's IR chances. "There's not much season left, either, so if it's a couple-, three-week thing, we'll just put him down. If he can get back next week or the week after, obviously, then you bring him back. I would say it's up in the air right now."
Harbaugh said Zbikowski is frustrated by his situation.
"Here's a guy, Zibby, that started for what, six games, and just played so well," Harbaugh said. "Then, boom, as soon as Ed (Reed) came back, he got hurt. Maybe it's fortuitous the way it worked out for us in that sense –- they weren't both hurt at the same time. But, (it's) really tough on Zibby. (I'm) just so disappointed for him because he was doing so well."
Also, running backs Le'Ron McClain and Willis McGahee missed Friday's practice because of the flu and are questionable. Cornerback Fabian Washington (hamstring), linebacker Tavares Gooden (calf) and wide receiver Derrick Mason (ankle) are probable.
The Associated Press contributed to this report.