Harbaugh, the current Michigan head coach, was the San Francisco 49ers' coach when Kaepernick was drafted by the team in 2011 and had his greatest success in the NFL.
"Anybody that talks to me is going to get a great recommendation that's going to knock their socks off," Harbaugh told the newspaper in a telephone interview.
Kaepernick's agents requested permission from the 49ers on Feb. 26 to seek a trade for their client. The 49ers are expected to keep Kaepernick on their roster past April 1, the date his $11.9 million base salary for 2016 becomes guaranteed, but will still look to trade the quarterback before or during the April 28-30 draft, sources have told ESPN's Adam Schefter.
One of the teams linked to Kaepernick in trade chatter, the Cleveland Browns, said on Monday via director of football operations Sashi Brown that rumors of their interest has been overblown. The Denver Broncos also have been mentioned as a possible destination, but coach Gary Kubiak, citing tampering rules, said "I can't talk about him" because "he's under contract with another team, so I can't talk about him."
Harbaugh lauded Kaepernick for his ability to play through injuries. The quarterback underwent surgery on a stress fracture in his leg after the 49ers drafted him, repairing an injury he suffered while playing for Nevada. Kaepernick also played through a foot injury in 2013, when he and Harbaugh reached the second of back-to-back NFC Championship Games.
"He's as tough as nails," Harbaugh told the newspaper. "That's a badge of honor in a lot of ways."
The 49ers quarterback is recovering from surgeries to his left shoulder, right thumb and left knee and chose to have private doctors perform his procedures rather than team doctors.
Harbaugh, who was fired as the 49ers' coach after the 2014 season, said he still texts with Kaepernick and said the quarterback has remained optimistic and focused.
"I've never been around him when he wasn't," Harbaugh told the newspaper. "As we talked about earlier, he's as tough as they come -- mentally tough and confidant as he should be."
If he stays with the 49ers In 2016, Kaepernick will be playing for his third coach since he joined the team in 2011, as former Eagles coach Chip Kelly was hired to replace the fired Jim Tomsula.
His career record as a starter is 27-20 with 56 touchdown passes and 26 interceptions, though he is coming off the worst season of his career, having been benched after eight games in favor of Blaine Gabbert.
In 2012, Kaepernick replaced an injured Alex Smith and helped the 49ers reach the Super Bowl, losing to the Baltimore Ravens 34-31. He threw for 50 touchdowns and rushed for 10 more with Harbaugh as his coach. He threw for a career-best 3,369 yards in 2014, Harbaugh's last season with the team.