"He's not going to wave fairy dust on it [and solve all the issues]," Fox said on his weekly coaches' show on WBBM Newsradio 780.
Fox is smart to somewhat temper expectations for Trubisky, who will be Chicago's first rookie quarterback to start a game since Kyle Orton in 2005.
In fact, the last quarterback whose first career start came on Monday Night Football was Colin Kaepernick against the Bears in Week 11 of 2012.
And the last first-year quarterback whose first career start came on Monday Night Football was Shaun King for the Buccaneers in 1999, according to ESPN Stats & Information. King and the Bucs came out with a 24-17 win over the Vikings.
Maybe Trubisky plays like Kaepernick did against Chicago five seasons ago -- 243 passing yards and two touchdowns -- but don't be shocked if Trubisky doesn't achieve that level of instant success.
The Bears have a shortage of weapons on offense.
Trubisky isn't walking into the same situation Dak Prescott did last season in Dallas.
Deshaun Watson -- picked 10 spots after Trubisky -- is off to an impressive start in Houston, but the Texans are a perennial playoff team.
DeShone Kizer -- drafted in the second round -- is off to a shaky start in Cleveland because the Browns, much like the Bears, have been perennial cellar-dwellers.
A rookie quarterback's supporting cast often determines his likelihood of success.
Since 2015, four quarterbacks taken in the top 2 started as rookies. They went a combined 16-35 with an average QBR of 46.6, according to ESPN Stats & Information.
Rookie quarterbacks under Fox have a combined 3-11 record. Jimmy Clausen was the last rookie to start for Fox in 2010.
Trubisky might end up being a product of his environment in Year 1, but Chicago still made the proper decision to start him.
Those four quarterbacks taken in the top 2 that struggled as rookies -- Jameis Winston, Marcus Mariota, Carson Wentz and the emerging Jared Goff. Things typically get better the year after the quarterback is in place.
Trubisky might only make the offense marginally better over the next 12 weeks, but the team's future still looks brighter than it did prior to the switch.