McDonald, 26, signed with the Dolphins in March and is suspended for eight games for violating the NFL's substance abuse policy.
Coach Adam Gase told reporters after the training camp session that Ajayi is being evaluated for a concussion. Monday was the first time this offseason that the Dolphins have practiced in full pads. Team personnel assisted Ajayi off of the practice field following the hit from the safety. Ajayi also took a big hit from linebacker Lawrence Timmons and defensive tackle Ndamukong Suh during the session, according to the Palm Beach Post.
Timmons also signed with the Dolphins in March.
Trainers took Ajayi's helmet while he was being escorted off of the field in Davie, Fla, according to the Miami Herald.
Ajayi rushed for 1,272 yards last season, good for fourth best total in the NFL. Kenyan Drake replaced Ajayi with the Dolphins' first unit.
"I'm sure he'll be back sooner rather than later," Drake told the Herald regarding Ajayi.
Miami has the day off on Tuesday. The Dolphins also have Damien Williams, Storm Johnson, Senorise Perry and De'veon Smith on their depth chart at running back.
T.J. McDonald on hit he had on Jay Ajayi today pic.twitter.com/3zSWkQc4cQ— Chris Perkins (@chrisperk) July 31, 2017
Ajayi, 24, should be considered a Top-5 running back this year in fantasy football. He is expected to see a bump in carries and receptions in 2017 and has said he never wants to" come off of the field."
The former fifth-round draft pick is entering his third NFL season as the Dolphins' "bell cow," according to Gase. He could be looking at 350 to 400 touches this season. Based on last season's production, if Ajayi had 400 touches, he would put up close to 2,000 yards from scrimmage (1,983). Ajayi averaged 4.96 yards per touch last season, with 260 rushing attempts and 27 receptions.