Although Reed underwent offseason hip surgery after signing a three-year, $15 million contract in March to join the Texans and has missed the first two games, the former NFL Defensive Player of the Year has been practicing for the past two weeks.
"We'll have to assume that he's going to play," Harbaugh said. "We'd be surprised if he didn't play in this game, and we'll have to assume that he's going to play the way he's played in the past."
Texans coach Gary Kubiak told Houston reporters Monday that Reed has been running full-speed in practice for two consecutive weeks and was extremely close to playing last weekend against the Tennessee Titans.
"We're going to listen to him, I say he's doing great," Kubiak said. "I think we're at a point right now where he's really strong in his movements and what he's doing and when he's ready to go, it's time to go. The arrow is pointing way up right now."
If Reed does play, it could change the complexion of the game by injecting one of the most instinctive safeties in NFL history into Sunday's matchup. Reed, 35, is a nine-time Pro Bowl selection and the NFL's all-time active leader with 61 interceptions for 1,506 interception return yards.
The Ravens won't have the benefit of having studied Reed playing for his new team. They do have extensive knowledge of his habits, including a penchant for gambling to make big plays and baiting quarterbacks into miscues.
"It's going to be like it was for other teams game-planning against us in the past," Harbaugh said. "It's a little tougher because we haven't seen him on tape, so we really don't know how he fits into their defense. We'll have to fit him into their scheme, which in a lot of ways is similar to what we've done here.
"It shouldn't be too hard to figure out where he's going to be. We'll just kind of envision him out there playing the way he's played for us all of those years."