UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques didn't complement each other, with frequent clinching disrupting the contest.
Two judges judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They prevail," Buatsi commented in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers felt the challenger, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Triumph in Slow Affair
His career development has plateaued in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who utilized a protective stance to barely take the cleaner stanzas.
The crowd's patience wore thin amid regular grabbing, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester fight to win the continental championship.
The venue fell silent for a deeply poignant homage before the fight as ten bells tolled in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a shout of "only one Ricky Hatton".
The fighting within the ring matched the emotion. Arthur was floored by a left hook squarely on the chin in the early round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was marked up soon, significant bruising forming under his eye as the duo traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the decision with scoring tallies of 114-114, close margin and wider gap, but the two athletes were determined and brave, everything their hero the legend stood for.
