UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the approaches didn't complement each other, with frequent clinching disrupting the action.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They prevail," the victor stated in the ringside as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
The Champion Earns Win in Slow Encounter
Buatsi's progress has slowed in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the better rounds.
The crowd's patience decreased amid regular grabbing, with the challenger falling to the floor several times during grappling.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester battle to capture the European light-heavyweight belt.
The stadium became quiet for a profoundly moving memorial ahead of the contest as ten bells sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a shout of "there's just one Hatton".
The fighting during the bout matched the sentiment. Arthur was floored by a left hook flush on the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was marked up quickly, heavy swelling forming under his eye as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor edged the decision with official scores of 114-114, close margin and 115-112, but the two athletes were relentless and brave, everything their inspiration Hatton embodied.
