Playing his first match for Hanazono Kintetsu Liners in Japan Rugby League One since being overlooked by Eddie Jones for last year's global showpiece in France, the 84-test veteran was sent to the sin bin in the 11th minute as his side slid to a 41-14 loss against Ricoh Black Rams Tokyo.
Former England loose forward Nathan Hughes opened the scoring for Black Rams with a try two minutes after Cooper's departure, a punishment for his side's repeated infringements.
The score had blown out to 21-0 by the time the 35-year-old returned, leaving his side a mountain it could not climb.
The Black Rams, who are coached by Australian Peter Hewat, scored three tries in each half, with Queenslander Isaac Lucas getting on the scoresheet in the second half.
Matt McGahan - the former Reds fullback and son of Kiwis rugby league legend Hugh - and former Wallabies prop Paddy Ryan also featured for the Black Rams in their first win of the season.
Two former coaches of Australia's national team endured contrasting afternoons.
An eye-catching result came at Kumagaya, where the Robbie Deans-coached Saitama Panasonic Wild Knights stormed home to beat Toyota Verblitz 43-27.
Saitama routed the visitors 35-0 in the second half, with the Australian-born Brave Blossoms pair of No.8 Jack Cornelsen and centre Dylan Riley among the try-scorers.
Verblitz, who are coached by former New Zealand boss Steve Hansen and featured All Blacks Beauden Barrett and Aaron Smith as well as Springboks legend Pieter Steph du Toit, arrived with high hopes.
The Wild Knights, conversely, were without two of their overseas stars in Wallabies winger Marika Koroibete and Springboks lock Lood de Jager.
Verblitz's confidence seemed justified as the visitors rocked last year's finalists with four first-half tries to lead 27-5 on the stroke of halftime.
Saitama have lost at home just once since 2019, and the second period showed why - Cornelsen beginning the comeback with a try in the 47th minute.
By the time Riley crossed for his team's fourth of the second period, the Wild Knights had gobbled up Toyota's lead.
They added a fifth before fulltime to complete a remarkable victory which keeps them top of the table.
One of Deans' successors in the Wallabies hotseat, Dave Rennie, had a less enjoyable afternoon as his Kobelco Kobe Steelers side featuring world player of the year Ardie Savea fell 44-36 to the Eddie Jones-advised Tokyo Suntory Sungoliath.
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