British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching disrupting the action.
Two judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, should have been awarded the victory but the contender endured the second setback of his 28-match professional journey.
The Champion Secures Win in Slow Affair
His career development has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who utilized a tight guard to narrowly win the better stanzas.
The spectators' acceptance wore thin during constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but many rounds were hard to judge. However, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific local rivalry battle to secure the European light-heavyweight title.
The stadium became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the stillness became a shout of "there's just one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the chin in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his face as the pair landed hard punches.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
The victor barely secured the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were determined and brave, everything their inspiration the legend embodied.
