British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a tight ten-round fight at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the approaches never quite clicked, with excessive holding marring the fight.
The officials had it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the decision but the British boxer suffered the second setback of his 28-match career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the better periods.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester contest to win the European light-heavyweight championship.
The arena became quiet for a profoundly moving tribute ahead of the contest as 10 bells rang in remembrance of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. The eventual winner was floored by a left hook directly to the chin in the early round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, noticeable damage forming under his eye as the fighters landed hard punches.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the verdict with judges scorecards of draw, narrow lead and 115-112, but each competitor were relentless and brave, exactly what their hero Hatton represented.
