Nagy said that Dalton will probably play at least the first quarter and a half but conceded "it can always change" based on game scenarios.
"If you get the ball or don't get the ball, that dictates some of it, which was the same thing last week," Nagy said. "But those two guys are going to get their fair share of reps as we go there with them."
After playing only the first two series in last Saturday's 20-13 preseason-opening win over the Dolphins, Dalton hopes to take advantage of his expanded playing time versus Buffalo.
"You just want to see the offense operate how it's supposed to," Dalton said. "I just want to be efficient and set the standard of how we're going to be getting in and out of the huddle, how we're going to move the ball and all that kind of stuff."
Dalton played just six snaps versus the Dolphins, completing 2 of 4 passes for 18 yards. Nagy said that the 11th-year pro would have played more if he hadn't already gained such a firm grasp of the playbook.
Fields, meanwhile, returned to practice Thursday after being held out of Wednesday's workout with a sore groin.
"We'll just be cautious with the reps," Nagy said before practice. "It's a [lighter practice] today, so there's not as many reps in practice today, so I think that'll be good for him to just get out there and kind of stay loose with that and feel good there."
Barring a setback, Nagy expects Fields to play against the Bills.
"I do, yeah," Nagy said. "Anything could happen, but I think yesterday giving him the day off to help him was good and he feels good. We wanted to be able to sacrifice that day of practice [Wednesday] to be able to get him right for Saturday."
In last Saturday's preseason opener, Fields completed 14 of 20 passes for 142 yards with one touchdown and a 106.7 passer rating.
Although Fields did not practice Wednesday, the first-round draft pick wore a helmet and remained engaged throughout the session.
"He's competitive," Nagy said. "He wants to be out there. That's the beauty of these guys, but at the same time, sometimes we've got to protect him from himself and make sure, and he understands it, he gets it.
"But I think Justin would tell you if we were in a situation where that was a game yesterday and he had to play, he would've played. But we wanted to be smart."
Nagy noted the excitement Fields showed when Dalton threw touchdown passes in 7-on-7 and 11-on-11 drills.
"Andy had an unbelievable day yesterday," Nagy said. "He was on fire yesterday, and so that was good to see. I thought it was pretty neat watching Justin in the background before the plays in the red zone putting up his hands for a touchdown because he saw [Dalton] was throwing the ball well."