UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the contest.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They win," Buatsi commented in the arena as he was greeted with boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts believed Parker, 31, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-fight career.
Buatsi Earns Win in Uneventful Match
His career development has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He started carefully, with neither fighter prepared to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the more effective rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger going down several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester fight to win the European light heavyweight belt.
The arena grew hushed for a deeply poignant memorial prior to the bout as ten bells tolled in honor of the city's native Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime disappeared, the quiet gave way to a roar of "only one Ricky Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling forming under his eye as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over twelve rounds.
The victor narrowly won the result with scoring tallies of draw, narrow lead and wider gap, but both fighters were persistent and full of heart, all the qualities their inspiration the legend embodied.
