Mostert made the announcement on social media, tweeting, "It's go time!!"
"Being told you may never step on the field again isn't easy to hear ... But when you bet on yourself, your faith, your determination, and modern medicine, good things happen. Hard work pays off! Beyond thankful!!" he added.
Mostert chipped cartilage in his left knee during the 49ers' first game of the season in 2021 and missed the remainder of the year, undergoing surgery in October. He signed a one-year contract with the Dolphins this offseason but did not participate in spring practices as he continued to recover.
First-year coach Mike McDaniel, who coached Mostert in San Francisco from 2017 to 2021, was noncommittal about Mostert's status for the start of the season when asked about him in May but insisted the team would not rush the eight-year veteran back.
"I know his expectations are to play Week 1 and we are not going to rush it," McDaniel said. "He's come too far to have a setback, so he's just diligently working day-in and day-out. But when he's ready, he'll be on the field for sure."
Known as one of the fastest players in the NFL, Mostert has struggled to remain on the field during the past two seasons, playing just nine of 33 possible games. When healthy in 2019 and 2020, however, he was one of the league's most efficient and dangerous runners; Mostert ranked second among running backs with a 5.37 yards-per-carry average and eighth in percentage of rushes of 10 or more yards (14.1%).
He joins what is now a crowded backfield in Miami that was in desperate need of an upgrade. Only the Texans and Falcons averaged fewer yards per game than the Dolphins last season, but the team signed former Cardinals running back Chase Edmonds and former first-round pick Sony Michel this offseason, along with Mostert.
While he's not necessarily expected to be Miami's feature running back, Mostert's experience with McDaniel's system should provide invaluable leadership in the Dolphins' locker room.
"I try to implement my role when I was with the 49ers, the room that was being built and the room that was built, we lean on each other," Mostert said in March. "It's not so much, 'Hey look, I'm going to beat this guy out. I'm competitive. I'm going to go out there and I'm going to show everybody that they made the wrong mistake by signing him and not signing me.' It's more so like 'Hey look, we're all brothers, we're in this together and this is how we are going to operate. I want to see you win and I hope that you want to see me win.'
"One thing that I even talked to the running backs coach [Eric Studesville] when I got in the facility, I basically broke it down to him and the mindset that I have for the running backs group is that I'm going to help you feed your family and you help me feed mine. That's something I've lived with going back to Coach [Bobby] Turner in San Francisco. He instilled that in the room. I'm going to be instilling that in the guys that aren't too familiar with the scheme and aren't too familiar with the playcalls and everything like that."
Mostert, 30, also said in March that he was "ahead of schedule" with his rehab and that his doctor called him the "fastest-healing patient that he's ever experienced."
His return will make Miami's new-look offense even more threatening, as both he and Edmonds are adept as runners and receivers out of the backfield.
"Yeah, I love the idea. I love the concept of me and Raheem in the backfield together," Edmonds said. "I was a big fan of his game unfortunately before he got injured and just to really see his explosiveness. Honestly he's probably the fastest running back in the NFL when he's at top speed, so I think it really brings a sense of explosiveness to the offense and being able to just capitalize on just us both having really good yards per carry averages. I think that's something that we can look to do to help this offense."