British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight ten-round bout at the Manchester venue.
But Parker found success, delivering effective single shots in a contest where the techniques never quite clicked, with frequent clinching marring the action.
Two judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the ring as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, should have been awarded the win but the British boxer endured the second setback of his 28-bout career.
The Champion Earns Win in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to barely take the more effective stanzas.
The audience's tolerance diminished because of constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester battle to secure the continental championship.
The stadium fell silent for a extremely emotional memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell disappeared, the quiet became a roar of "only one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was floored by a powerful punch flush on the face in the second round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up soon, heavy swelling appearing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the decision with official scores of even score, 115-113 and clearer margin, but the two athletes were relentless and brave, all the qualities their hero Hatton stood for.
