Fields has missed the Bears' past two games with an injury to his ribs, but Nagy said Wednesday that the rookie has been medically cleared.
Nagy said Fields' fractured ribs have fully healed.
"This comes down to, for him -- like I've been saying the whole time -- it's a pain deal that we can't really simulate. We go off what he says; we go off what our doctors say. When he's fully cleared like he is, then we're ready to rock and roll. He's the starter when he's healthy, and that's where we're at," Nagy said.
Fields said Wednesday that he feels good and is ready to play.
"I mean, yeah, there's pain there, but I mean, the pain's just not unbearable," Fields told reporters. "I know there's gonna be pain there but I mean, it is what it is. You have three cracked ribs, of course there's gonna be pain. As long as it's just not crazy pain where I can't bear it, I'm gonna play and practice."
Nagy said that the decision to start Fields wasn't based on Dalton's injury and that he isn't sure whether Dalton or Nick Foles will be the backup to Fields for Sunday's game.
Fields, the No. 11 pick in this year's draft, has completed 58.1% of his pass attempts this season, throwing for 1,361 yards with four touchdowns and eight interceptions in 10 games. He also has rushed for two scores this season.