The Boise State tailback tumbled down draft boards amidst concerns about knee issues. Ajayi had knee surgery in 2011.
NFL Media's Mike Mayock said several teams told him they took Ajayi off their board due to the knee, which was characterized as bone rubbing on bone.
"One team will say, 'He's off the board because he's bone on bone,'" Mayock said. "Other teams say, 'That's fine. We'll get one contract out of him.' "
Ajayi became the 14th running back drafted in 2015.
Despite the medical concerns, Ajayi answered the bell every week for the Boise State Broncos, becoming the only player in FBS history with 1,800 yards rushing and 500 yards receiving in a single season.
The north-south runner shows good patience while combining solid power with slipperiness and a good second gear.
The Dolphins have been looking for someone to take some of the work off Miller's shoulders. Expect Ajayi to earn that role quickly as a rookie.
Even if the running back's knee doesn't allow him to get to a second contract, in the fifth round Miami nabbed a player with the talent to compete and contribute at the pro level from Day 1.