Arnold will miss the Brumbies' next two games and the Wallabies' first Test against Ireland on June 9, casting doubt over his involvement in the rest of the series.

Arnold was sent off after a high tackle on Johannesburg Lions playmaker Elton Jantjies in the Brumbies' 42-24 loss at Ellis Park on Sunday morning Australian time.

The judiciary panel intially hit Arnold with a six-week ban, but he had his punishment reduced to three weeks because of his previous clean record.

The Brumbies have flown reinforcements to South Africa for their clash against the Pretoria Bulls this week, calling on hooker Connal McInerney and lock Darcy Swain for back up.

Teenage back-rower Rob Valetini suffered another knee injury and is flying back to Canberra alongside teammate Robbie Abel, who injured his calf before kick-off against the Lions.

Arnold has been in career-best form and was set to secure a Wallabies spot, but won't be able to play for the Brumbies again until after the mid-year Super Rugby hiatus.

He pleaded guilty to the high shot in the 60th minute, which proved the tipping point in the Brumbies falling to their fifth consecutive defeat.

"Having conducted a detailed review of all the available evidence, including all camera angles, and a statement from the player, the foul play review committee upheld the citing under law 9.13," the committee said on Sunday night.

"With respect to sanction the foul play review committee deemed the act of foul play, a dangerous high tackle, merited a mid-range entry point of [six] weeks.

"However, taking into account mitigating factors including the player's excellent, clear, disciplinary record, his expressed remorse and his guilty plea at the first available opportunity, the foul play review committee reduced the suspension to [three] weeks.

Arnold will be free to return to the field after June 9, which will be the start of the Test series against Ireland. He will be available to play in the final two Tests and the last three games of the Super Rugby regular season.

The giant second-rower spent time in the sin bin against the Lions after giving away a maul penalty, but his game took another turn when his shot got Jantjies high.

The Brumbies were leading 24-21 when Arnold was sent off and then reduced to 13 men when second-row partner Sam Carter was shown a yellow card for repeated maul infringements.

"It probably was [the killer]. We were leading at that point and had to go on with 13 men. Even then we were making inroads with our attack," said Brumbies coach Dan McKellar.

"But the result turned the way it did because we played with 13 men for 10 minutes and [Arnold was off] for the final 20 minutes. It was a task that was too big for us."

There was confusion about the circumstances surrounding Arnold's red card and whether referee Nick Briant deemed it a red card offence or sent Arnold off because he was already in the change rooms.

Arnold left the field for a head injury assessment before Briant made a decision on the penalty.

The inconsistency of the SANZAAR judicial panel will give Arnold a nervous wait, in particular because the Brumbies have already felt the wrath of the judiciary this year.

Brumbies winger Chance Peni was suspended for seven weeks for a high tackle on Canterbury Crusaders flyer Israel Dagg at the end of April.

Peni's ban was increased by one week because of his previous bad record, meaning his punishment was worse than the one given to Bulls prop Pierre Schoeman, who pleaded guilty to biting.

Melbourne Rebels back-rower Amanaki Mafi was given a yellow card for a high shot that knocked out Brumbies prop Scott Sio two weeks ago, but Mafi escaped any further punishment.

Crusaders prop Joe Moody was suspended for just two weeks for striking Kurtley Beale with an elbow to the throat.

The Brumbies will be hoping Arnold's clean record will work in his favour as they fight for a breakthrough win and to restore pride in their season.

But they will have to fix their discipline before they play the Pretoria Bulls at Loftus Versfeld on Saturday night after giving away 20 penalties against the Lions, including a red card and two yellow cards.

The Bulls will be desperate to make amends as well after being beaten by the Argentina Jaguares 54-24 in Buenos Aires on Sunday morning.

"[I was] really, really proud of the team's effort tonight. It's always tough to win when you're down a man and against a Lions team that's hot at home," said Brumbies captain Christian Lealiifano.

"We wanted to take our game forward and we felt like we made some good steps. We've just got to do it more consistently with 15 guys [on the field].

"I think we'll take a lot of confidence out of the team's effort, we'll look at it and go again next week."

SUPER RUGBY ROUND 15

Saturday: Pretoria Bulls v ACT Brumbies at Loftus Versfeld, 11.05pm. TV time: Live on Fox Sports.