Heap has been battling a hamstring injury for the past two weeks. The injury kept him out of Monday's game against the Texans and will likely hold him out Sunday, as well.
Zbikowski suffered a back injury in the first half against Houston and has not been able to return to team drills since then.
"I think it looks bad for Zibby," Harbaugh said. "Todd, probably less than 50-50. He'll probably be questionable or doubtful. Hamstrings, he's fighting like crazy and it might feel great and it might not. We'll see."
Zbikowski's injury appears to be the more serious of the two at this point, and while Harbaugh said that he doesn't think that the team will have to put the third-year safety on season-ending Injured reserve, it seems like that could be an option if doctors feel like he won't be able to return within the next week or two.
"When you've got a back (injury) you just have to see," Harbaugh said. "He's got an MRI coming back on that. We'll just have to see. There's not that much season left. If it's a couple, three-week thing, then we're just going to put him down. If he can get back next week or the week after, then obviously we'll bring him back. I would say it's up in the air right now."
"Here's a guy who started for six games and just played so well and now boom as soon as Ed came back, he got hurt," Harbaugh added. "Maybe that's fortuitous the way that worked out. It's really tough on Zibby. I'm so disappointed for him because he was playing so well."