British light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker found success, landing some telling clean blows in a fight where the styles didn't complement each other, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor commented in the ring as he was met by disapproval from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Match
Buatsi's progress has plateaued in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers willing to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to barely take the cleaner stanzas.
The audience's tolerance wore thin amid regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight title.
The venue grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of Manchester's own Hatton, who passed away at age 46 in September.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence became a shout of "only one Hatton".
The fighting inside the ropes equaled the emotion. Arthur was floored by a left hook directly to the jaw in the initial stage, rising shakily to his position on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured early, heavy swelling developing under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of even score, close margin and clearer margin, but each competitor were relentless and courageous, everything their inspiration the legend represented.
