Veteran Nick Foles is not under consideration to start, according to Nagy.
"We know what we are doing and that's what we are believing in and going with," Nagy said after Friday's practice.
Nagy has repeatedly insisted that Dalton is Chicago's starter when healthy, but the 33-year-old quarterback suffered a bone bruise in his left knee in Week 2 and was inactive for Sunday's 26-6 loss to the Cleveland Browns.
Dalton had limited participation in practice all week and is officially listed as questionable to face the Lions.
"I feel good," Dalton said Friday. "When you suffer an injury, you're trying to do everything you can to rehab and get feeling good as soon as possible. So everything's been heading in the right direction and everything's improving so I feel good about where I'm at."
Fields suffered a right thumb injury against the Browns, but had full participation all week and is not listed with an injury designation on the final report.
Cleveland sacked Fields nine times in the rookie's NFL starting debut and held him to just 6 of 20 passing for 68 yards and 12 rushing yards on three attempts.
For his part, Fields said he enjoyed a solid week of practice and successfully put the Browns game behind him.
"This week's practice compared to last week's practice, I felt just more comfortable at practice just kind of getting that under my belt and getting kind of past that," he said. "Last week, you really can't get worse than that. I mean, I hope not. But I felt comfortable this week."
Nagy echoed the sentiment.
"Justin's game that he played last week, he did a lot of things that he's going to learn from," Nagy said. "But we also, and myself, have to help him. I think the biggest thing for him is, 'OK, here's how I was in the week of practice. Here's how I was on game day, both physically and mentally. Now how am I going to be heading into this week? Am I getting better? Is there improvement? Am I getting better at the things I was good at? Am I getting better at the things that I wasn't so good at?'
"And that's what we're doing as coaches throughout the week of practice, is, being able to watch him and see that. I can say from last week to this week that you do see that advancement. That there, knowing and feeling how he practiced, is why I would answer it that way."
In other news, Nagy declined to reveal whether he would continue to call offensive plays or hand the duty over to offensive coordinator Bill Lazor or another assistant coach. The Bears had just 47 total net yards, 1 net passing yard and averaged 1.1 yards per offensive play in Sunday's ugly defeat to the Browns.