Kindle, a linebacker from the University of Texas, fell down two flights of stairs in July. There was recent optimism that Kindle could return later this season, but neurological tests were not promising enough, so the Ravens will be cautious about Kindle making a return to football.
"He's got some serious issues that he's dealing with. I think the chances of him playing this year are slim," Harbaugh told reporters in Baltimore on Friday. "We'll be very, very careful. The doctors will be very careful. Sergio will be very careful with that type of an injury."
Kindle, a former Big 12 standout, said it's just a matter of time before he returns to football.
"I know I'll play football again," he said. "I don't know when, but I'm going to make it back."
Kindle was Baltimore's second-round pick in April, taken No. 43 overall. The Ravens are still expected to sign him to a rookie contract.
"We're going to try and sign him to some form of a deal that reflects the situation and give him an opportunity," Harbaugh explained. "Again, I highly doubt he'll play this year, but it will be an opportunity for him to get on track for the future. We want to give him a chance to pursue his dream still."
However, because of the nature of the injury, there's no guarantee Kindle will make it back.
"There's no way to predict these things," Harbaugh said. "You don't know how the brain heals. It's different in everybody. We'll just see how it goes."
Before Friday's announcement, Kindle had said Thursday that he's remaining patient as he waits for the Ravens to talk with the NFL Management Council before proceeding with contract talks.
"They're supposed to do that before they can talk to me," he said. "I hope we're talking soon about me coming back there and doing my contract. I don't know when that will happen, hopefully soon."
Kindle said he continues to make steady progress and is working out hard.
"I'm feeling good," he said. "I'll be feeling a lot better once I get back there."
Kindle said he remains upbeat about his outlook.
"I'm not frustrated," Kindle said. "At first, I was because I didn't understand and now I understand. Being patient is a good thing. Being frustrated won't help. I'm waiting for the good word."
James Walker covers the AFC North for ESPN.com. Information from The Associated Press contributed to this report.