"He's [Foles] our starter moving forward," Nagy said on a Zoom call.
Nagy also said Monday that tests confirmed that running back Tarik Cohen has a torn ACL and will miss the remainder of the season.
Foles, 31, replaces 2017 second overall pick Mitchell Trubisky, whom Nagy benched in the third quarter with Chicago trailing by 16 points to the Atlanta Falcons on Sunday. Foles responded by throwing three fourth-quarter touchdown passes, including the game-winning 28-yard strike to Bears wide receiver Anthony Miller just after the two-minute warning in Chicago's 30-26 come-from-behind victory.
The Bears (3-0) host the Indianapolis Colts (2-1) in Week 4.
"We got up this morning and went through the tape, and at the end of it we decided that we're going to start Foles as a starter against Indianapolis," Nagy said. "Both quarterbacks are aware of that. For us it's never easy when you go into these type of situations. I want to credit both of those guys in yesterday's situation for being really supportive of each other this whole time. Even as hard as that was for Mitch to be told that Nick's going to go in and play, Mitch was to his credit just an awesome supporter of Nick the rest of that game."
Cohen suffered his injury on a punt return when Atlanta's Brian Hill got pushed into the back of his right leg, causing his right knee to bend awkwardly. Cohen went to the ground and immediately clutched his knee. The 5-foot-6 all-purpose threat was unable to put any pressure on his right leg when trainers helped him off the field.
He had two carries for 21 yards and three receptions for 20 yards before the injury. The Bears signed Cohen to a three-year extension earlier this month.
Foles had two additional touchdown passes overturned by the officials.
"I just wasn't expecting this today," Foles said after the game. "Obviously we went out there today and got a win as a team. I felt good out there. Not perfect. But I felt good."
Trubisky went 13-for-22 for 128 yards, one touchdown and one interception (71.8 passer rating). The Bears were an ineffective 1-of-8 on third down, with several of Trubisky's downfield passes being off the mark. Trubisky overthrew a wide-open Miller on what would have been a touchdown completion before halftime.
"We just missed it," Nagy said of the Miller incompletion. "Just like he [Trubisky] said after the game yesterday, you have to complete that ball."
Trubisky, whom Chicago selected second overall in the 2017 NFL draft, ahead of Patrick Mahomes and Deshaun Watson, led Chicago to a 2-0 start this season. He rallied the Bears against the Lions in Week 1 when he fired three fourth-quarter touchdown passes to complete a 27-23 comeback win. Trubisky, 26, was largely ineffective for the first three quarters at Ford Field before the late heroics.
"The situation sucked, but it was just the flow of the game and how it happened and Coach made the decision that he felt was best for the team," Trubisky said Sunday of getting benched. "I'm really happy for this team. They battled back and it was awesome to get a W, but it sucks to get news like that."
In 2018, Nagy's first season as head coach, the Bears went 12-4 as Trubisky passed for 3,223 yards and 24 touchdowns with 12 interceptions and rushed for 421 yards and three scores. He finished third in Total QBR (71.0).
In 2019, Trubisky, while playing much of the season with a shoulder injury that required surgery in January, ranked 28th in the NFL in Total QBR (39.4) and tied for 27th in touchdown passes (17), 21st in passing yards (3,138), 32nd in yards gained per pass attempt (6.1) and 28th in traditional quarterback rating (83.0) as the Bears finished 8-8.
Chicago traded for Foles -- the MVP of Super Bowl LII while with the Philadelphia Eagles -- at the onset of the new league year. The Bears later declined Trubisky's fifth-year option and announced an open quarterback competition in training camp.