While the plan is for Fitzpatrick to start Friday against Tampa Bay, coach Brian Flores said the competition for the starting quarterback spot remains "fluid."
Rosen's reps with the starters is a new development and one in the right direction of the former Arizona Cardinals first-round selection garnering the job.
"I thought it was a lot of fun," Rosen told the media following practice via Dolphins.com's Alain Poupart. "Got a lot to study tonight, overall it was a positive."
Following Tuesday's practice, Flores was far from glowing about Rosen, saying he did an "OK job" and needs to improve his body language.
If the plan to start Fitzpatrick on Friday holds, the game could mirror the team's first preseason outing in which the veteran started and played two series before Rosen came on and played all the way into the fourth quarter.
While the competition is no doubt amping up, Rosen is heaping praise on Fitzpatrick, who's been in a situation similar to this just last year when he was with Jameis Winston in Tampa Bay.
"He's been unbelievably helpful," Rosen, who's on his second team in two seasons, said of Fitzpatrick, who's playing for his eighth squad in 15 seasons. "Probably one of the best mentors I've ever had in this sport."