UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close 10-round fight at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the techniques never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi stated in the arena as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the victory but the British boxer experienced the second loss of his 28-match career.
Buatsi Claims Triumph in Slow Match
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers ready to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner stanzas.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur overcame Rea by divided decision in a terrific local rivalry contest to capture the European light-heavyweight belt.
The arena grew hushed for a deeply poignant tribute before the fight as 10 bells tolled in remembrance of the city's native Hatton, who succumbed at age 46 in recently.
It was the premier televised fight night in the location since Hatton's passing. As the final chime faded, the silence gave way to a roar of "there's only one Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, noticeable damage forming under his orbital area as the pair traded hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but each competitor were relentless and full of heart, all the qualities their inspiration the legend embodied.
