UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a fight where the techniques failed to mesh, with frequent clinching spoiling the action.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor said in the ring as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, merited the win but the contender experienced the second loss of his 28-match career.
The Champion Earns Win in Dull Affair
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance decreased amid frequent holding, with Parker going down multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester contest to win the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as a bell ceremony rang in honor of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime ended, the stillness transitioned to a roar of "there's just one Hatton".
The action within the ring reflected the emotion. Arthur was floored by a powerful punch squarely on the face in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged quickly, noticeable damage forming under his face as the duo landed punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and clearer margin, but both fighters were determined and full of heart, everything their hero the legend embodied.
