UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing 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 bout at the Manchester venue.
But Parker found success, landing some telling individual punches in a contest where the styles didn't complement each other, with frequent clinching disrupting the contest.
The officials judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the arena as he was greeted with boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Dull Match
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a solid defense to barely take the cleaner rounds.
The crowd's patience diminished amid regular grabbing, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester fight to capture the European light-heavyweight belt.
The venue fell silent for a extremely emotional homage before the fight as ten bells sounded in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence became a shout of "there's just one Ricky Hatton".
The action within the ring equaled the feeling. The eventual winner was floored by a left hook flush on the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising developing under his orbital area as the duo traded powerful shots.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur edged the result with judges scorecards of even score, narrow lead and wider gap, but the two athletes were determined and courageous, all the qualities their hero the legend embodied.
