British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the boxing arena.
But Parker found success, delivering effective clean blows in a fight where the techniques never quite clicked, with constant grabbing disrupting the action.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," the victor stated in the ring as he was greeted with boos from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, 31, should have been awarded the victory but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Affair
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The audience's tolerance wore thin during regular grabbing, with the challenger going down several times during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Inspires Local Rivalry Contest
In the co-main event, Arthur overcame Rea by split-decision in a terrific all-Manchester battle to secure the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute before the fight as a bell ceremony sounded in remembrance of Manchester's own Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime ended, the stillness gave way to a shout of "only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was knocked down by a significant blow flush on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising developing under his eye as the duo traded punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the result with scoring tallies of even score, close margin and wider gap, but each competitor were determined and brave, everything their hero the legend embodied.
