Initial tests did not reveal any broken ribs, a source told ESPN's Adam Schefter. Fields will undergo further tests Monday.
Fields walked back to the locker room early in the third quarter after first being examined in the blue medical tent. It's unclear on which play he suffered the injury; the first-round draft pick was hit often by the Baltimore defense.
Fields took a sack on his final drive but stayed in the game for four more plays.
After the game, coach Matt Nagy said he was unsure whether Fields had broken ribs, or if the injury was a result of one particular hit or a cumulative effect.
"I have no idea," Nagy said. "I'll have to find out. We'll start getting into some of that later on."
Fields finished the afternoon 4-of-11 for 79 yards with a lost fumble.
Veteran Andy Dalton replaced Fields and promptly threw a 60-yard touchdown pass to wide receiver Darnell Mooney. Dalton lost the starting job to Fields after he suffered a knee injury in Week 2 against the Cincinnati Bengals.
He also threw a 49-yard touchdown pass to Marquise Goodwin in the fourth quarter and finished the game 11-of-22 for 201 yards.
The Bears (3-7) have a quick turnaround and play on Thanksgiving Day in Detroit against the Lions (0-9-1).
Dalton said he's prepared to start if Fields' ribs render him unable to play.
"I prepare the same way I've prepared this whole year," Dalton said. "So if I'm starting I'll be ready to go."