The NFL Insiders crew breaks down the rulings that led to fines for Broncos' players Darian Stewart and Brandon Marshall for illegal hits on Cam Newton. (1:05)

ENGLEWOOD, Colo. -- Denver Broncos safety Darian Stewart and linebacker Brandon Marshall will have to open their checkbooks in the wake of the team's 21-20 victory over the Carolina Panthers in last Thursday's NFL season opener.

Stewart has been fined $18,231 by the NFL, sources told ESPN, for his helmet-to-helmet hit on Panthers quarterback Cam Newton that came with just under 40 seconds remaining in the game. Stewart was flagged for roughing the passer on the play, but Newton was also flagged for intentional grounding on the same play, offsetting the penalties.

Marshall, whose hit in the third quarter was not flagged, was fined $24,309 for impermissible use of a helmet, a source told ESPN.

"That's a huge fine, I know what it looks like, it looked bad, but that's a lot of money, so yes I'm upset about it," Marshall told reporters on Wednesday.

Replays clearly showed the contact, but Stewart said after the game that he believed he was playing within the rules.

"I thought I got him with my shoulder, too,'' Stewart said. "But he's a hard player to tackle, he's so big, and when he runs, he leans like a running back, coming right at you.''

Panthers quarterback Cam Newton received at least four helmet-to-helmet hits -- of which only one was called for a penalty -- during Thursday's loss to the Broncos. AP Photo/Joe Mahoney

Stewart and Marshall's fines raise the Broncos' season total by defensive players to three. Cornerback Bradley Roby was docked $24,309 for unnecessary roughness as a result of a hit on Los Angeles Rams wide receiver Duke Williams in a preseason game last month.

And more Broncos could be added to the list, given the league was reviewing several hits on Newton from the opener, including contact made on other plays by linebacker Von Miller and Roby.

NFL vice president of officiating Dean Blandino said Tuesday he believed one penalty had not been flagged for a hit on Newton in the game that should have been. That play is believed to be Marshall's hit.

Blandino, who issues a weekly highlight tape to officiating crews with points of emphasis for the upcoming week, included the Stewart and Marshall hits on this week's tape in an effort to help officials distinguish what's legal and what's not, according to ESPN NFL Insider Ed Werder.

Several members of the Panthers and Newton's father, Cecil, said the reigning NFL MVP was treated unfairly by the officiating in the game.

The NFL and the NFL Players Association each said earlier in the week they would investigate how the concussion protocol was enacted in Newton's case to see if the quarterback was checked quickly and thoroughly enough during the game.

Broncos coach Gary Kubiak has defended his players against criticisms that the team is playing "dirty.''

"I disagree," Kubiak said Monday. "We play hard, we're going to continue to play hard.''

Said Marshall: "We're not dirty. When I think of dirty, I think of stepping on somebody's ankle at the bottom of the pile, twisting somebody around. We just play hard, man. We hit hard. We play fast ... we play faster than any defense in the league, in my opinion. Look at some of the dirty plays in the league. There's a lot of dirty players. That's not our M.O.''

The Broncos weren't the only team to be fined. Panthers guard Trai Turner was also fined $9,115 for taunting, a source told ESPN NFL Insider Adam Schefter.