On the play, Campbell was rushing off the right edge, and Broncos left tackle Ryan Clady stood in a pass-block stance but had yet to make contact with Campbell. At the snap, Thomas, who was lined up next to Clady, dived into Campbell's knees for the chop, which caused the injury and forced Campbell to miss most of the second half.
Thomas was flagged for the hit, which left Campbell with an MCL strain that could sideline him for up to three weeks.
On Monday, the NFL said the officiating crew in Denver made the right call.
"The play was penalized because it was an illegal chop-block, a violation of Rule 12, Section 2, Article 3 items c) and d)," NFL spokesman Michael Signora said. "The officials threw the flag for the 'reverse chop.' Thomas blocked Campbell 'in the area of the thigh or lower,' as specified by rule, and Clady engaged Campbell high 'simultaneously or immediately after the block' by Thomas, resulting in the penalty.
"The play also violated part c) of the rule, known as a 'lure.' While Thomas chops Campbell, Clady 'confronts the defensive player in a pass-blocking posture but is not physically engaged with the defensive player.'"
After Sunday's game, Cardinals coach Bruce Arians called the chop-block the dirtiest play he's seen in 37 years of coaching. He later said the hit was deliberate and thought Thomas should be benched until Campbell returns.
"You'll never convince me it wasn't premeditated," Arians said Monday. "The one guy set him up."
Thomas previously expressed remorse for hurting Campbell.
"The main thing is there was no intention for anybody to be hurt on the play, and hopefully he'll be back soon," Thomas said.
ESPN.com Broncos reporter Jeff Legwold and ESPN.com Cardinals reporter Josh Weinfuss contributed to this report.
Arians Calls Thomas' Chop Block 'Premeditated'
ESPN Cardinals reporter Josh Weinfuss discusses Broncos TE Julius Thomas' chop block on Cardinals DE Calais Campbell, which Arizona coach Bruce Arians believes was premeditated. According to Cardinals GM Steve Keim, Campbell suffered a Grade 2 MCL sprain