Heap, who's fourth on the team with 37 catches for 546 yards and five touchdowns, suffered the injury during last Sunday's 13-10 loss to the Pittsburgh Steelers. Rookie Ed Dickson, a third-round draft pick out of Oregon, is expected to start in Heap's place.
"I would say, realistically, it'd probably be tough for him to get back," Harbaugh said. "I'll hold out hope as always, because he's a tough guy and he's been rehabbing it like crazy. And we made a lot of progress from yesterday to today, but we'll probably plan for the worst and hope for the best."
Harbaugh also noted the differences in the Ravens' game plan with Dickson, not 10-year veteran Heap, at tight end.
"There's probably some subtle changes," the coach said. "You might not do as much with Ed as you would do with Todd. ... (Todd is) really an accomplished player. He can do so many things. He's very versatile.
"Ed is in his first year, so I think we have to keep that in mind. But Ed is capable of doing some things that are special. He's fast, he's athletic, he'll get in there and fight you blocking, so we've got a lot of confidence in he and Dennis (Pitta)."
Rookie kick returner David Reed, whose home was raided by police Tuesday night in a narcotics investigation, will play, Harbaugh said. Reed hasn't been charged with a crime in the case.
"It's not something that really affects his football status, certainly at this time," Harbaugh said. "He'll have a chance to tell his side of it in the proper way when the time comes, and we'll just have to see how it all shakes out. It's in other people's hands right now, and I think time will kind of work that out as we go."