Facing a potential must-win clash with Cronulla next weekend, Canberra were on Sunday told Horsburgh had been picked up for a 79th-minute hit on Broncos forward Corey Jensen.
The incident went unpenalised on field but was on Sunday morning deemed serious enough to attract a grade-one charge from the match review committee.
Players pulled up for a similar incident would usually be able to escape with a fine but the Raiders firebrand's poor disciplinary record means he will now miss three matches with an early guilty plea - or four if he fights the charge unsuccessfully.
Winger Jordan Rapana was also charged - for a trip on Selwyn Cobbo that had him sent to the sin bin - but can accept a $3000 fine that would allow him to face Cronulla on Sunday.
The latest developments will do little to please an already frustrated Raiders.
A win against the Broncos would have been enough for Canberra to wrap up a top-eight spot but coach Ricky Stuart was left fuming over several key calls in the 29-18 loss.
Stuart, who has racked up $160,000 in fines throughout his career, claimed after the match he was "p***ed off".
The Raiders' main issue was with a late escort call against Hudson Young that killed off the hosts' last hopes of winning the game.
Canberra must now hope results fall in their favour over the final round of the season, or else face a do-or-die clash next Sunday against the Sharks.
Captain Elliott Whitehead, who struggled to hide his disappointment with Saturday's refereeing, insisted he saw positive signs in his side's performance.
"I try not to get frustrated with referees but some of the calls tonight (I'm) frustrated about," he said.
"It's probably best not to say anything because I don't want a fine out of this.
"We wanted to win tonight and we knew we were capable of winning the game and we showed that tonight.
"We played really well. We didn't get some calls our way but apart from that (we're) pretty pleased with how we played.
"But still we didn't get the two points, that's the disappointing thing. We'll have to go back to work and we need to win next week now."
Playmaker Jack Wighton agreed, suggesting the attacking flair and defensive grit the Raiders had shown would hold them in good stead.
"We've just got to drop that game, take the good out of it and keep going about our business," he said.
"Nothing else matters; what's been is done and we roll forward."
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