Mills bumped Charlie Spargo in the third term of Sydney's heavy loss to Melbourne on Sunday at the MCG, which left last year's grand finalists in trouble sitting two games plus percentage outside the top eight with a 4-7 record.
It was Mills' first AFL game since last year's qualifying final.
Mills was airborne when he bumped Spargo, who had to leave the field afterwards for a head injury assessment.
The incident was graded as careless conduct, medium impact and high contact.
Sydney, who have a must-win home game on Saturday night against Adelaide, have until Tuesday to decide whether they take the charge to the tribunal.
There will be at least tribunal case this week, with Fremantle to challenge captain Alex Pearce's three-game rough conduct ban incurred in their win over Port Adelaide on Saturday.
Mills was among 13 players charged from Sunday's games and the only suspension.
Star West Coast recruit Liam Baker escaped with a fine for driving his forearm into the back of James Peatling as the Adelaide midfielder was on the ground.
Baker was charged with "other misconduct" and can accept a $3125 penalty.
The Eagle was also among seven players fined for the melee that immediately followed the incident.
Adelaide captain Jordan Dawson was also fined for striking West Coast young gun Harley Reid.
Meanwhile, Fremantle coach Justin Longmuir continues to strongly defend Pearce, having backed his skipper post-game.
While Longmuir has sympathy for Darcy Byrne-Jones, in his opinion the collision that left the Port Adelaide player concussed was a football act.
"His (Pearce's) view is he's hard done by and very disappointed," Longmuir told Fox Footy's AFL360 program.
"I was so strong on it after the game because I didn't think Alex turned and braced to bump. If anything, Alex left himself open as well."
Pearce's teammate Corey Wagner said earlier on Monday that Pearce would have been criticised had he pulled out of the incident.
Byrne-Jones was running with the flight of the ball and was concussed when Pearce collected him in Saturday night's home Fremantle win.
Pearce was airborne and took his eyes off the ball just before contact, but did not turn his body in a bumping motion.
"It's a contact sport. You've got to brace for those positions. And if he pulls out, he gets criticised for not being hard enough. It's a moment-by-moment sort of thing," Wagner said.
"They're hard ones. It's the sport we play. Some might get weeks, and some you might not. Hopefully, Alex gets off, but if not, you've got to keep it consistent."
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