UK light heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker found success, delivering effective single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the contest.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor said in the ringside as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the win but the contender suffered the second loss of his 28-fight career.
The Champion Secures Triumph in Uneventful Affair
The fighter's advancement has slowed in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to narrowly win the cleaner stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger going down several times during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester fight to secure the European light-heavyweight belt.
The venue grew hushed for a deeply poignant homage before the fight as 10 bells sounded in honor of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell faded, the silence gave way to a chant of "there's just one Hatton".
The fighting within the ring equaled the sentiment. Arthur was sent to the canvas by a significant blow directly to the jaw in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was injured early, heavy swelling appearing under his orbital area as the duo traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the result with judges scorecards of 114-114, 115-113 and wider gap, but both fighters were determined and brave, exactly what their idol Hatton stood for.
