British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective clean blows in a fight where the styles never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," the victor said in the ring as he was faced boos from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, merited the victory but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to edge the more effective periods.
The audience's tolerance diminished amid constant clinching, with the challenger falling to the floor various instances during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry battle to secure the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in honor of local hero Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the quiet became a chant of "there's just one Ricky Hatton".
The action inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a significant blow flush on the chin in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising appearing under his eye as the pair landed powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were determined and brave, everything their idol Hatton represented.
