Lewis missed last Sunday's game against the Bengals because of a toe injury, but he returned to practice this week.
Asked how it's been to get Lewis back on the field the last few days, Harbaugh simply replied: "It's been good."
Harbaugh was then asked if he had any expectations for Lewis in regards to Thursday night's game against the 49ers.
"None that I would share," Harbaugh said. "I really don't know. It's not like we've got some secret that we're holding back from anybody."
Harbaugh said that a number of players are battling various injuries right now, and could be considered gametime decisions due to the short week.
"We've got three, four, five guys that are up in the air," Harbaugh said. "Our biggest issue right now is nicks and bumps and bruises. If you want to make a comparison, they're healthy. We're not quite as healthy right now, and that's something we'll be crossing our fingers for (with) different guys like that."