Lamar Jackson fumbled 12 times last season, which led the NFL by a wide margin.
The quarterback fumbled three more times in the Baltimore Ravens' playoff loss to the Los Angeles Chargers.
Jackson told reporters on Tuesday that he's "fixed" the problem and that his only concern for the regular season is winning.
"I think it's been way better. Just watching the practices, you probably saw it," Harbaugh said. "Lamar makes very few mistakes with ball handling right now. The snaps have been much more accurate than they have in the past, so that starts everything off on the right foot. You haven't seen a lot of the more complicated ball-handling plays, but the ones we have run have been well done."