UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
The officials scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi stated in the arena as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts felt the challenger, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The spectators' acceptance wore thin because of frequent holding, with the challenger going down several times during grappling.
The underdog closed well, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to secure the European light-heavyweight belt.
The arena became quiet for a extremely emotional memorial prior to the bout as 10 bells tolled in memory of the city's native Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound ended, the quiet became a chant of "there's just one Hatton".
The fighting within the ring matched the sentiment. The eventual winner was knocked down by a left hook flush on the face in the second round, rising shakily to his footing on the eight count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged quickly, noticeable damage developing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were determined and brave, all the qualities their hero the legend embodied.
