UK light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive 10-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the approaches didn't complement each other, with excessive holding disrupting the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They prevail," the victor commented in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Victory in Uneventful Encounter
The fighter's advancement has stalled in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to edge the better periods.
The crowd's patience diminished during frequent holding, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester battle to capture the European light heavyweight belt.
The arena grew hushed for a extremely emotional homage before the fight as ten bells rang in memory of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the last bell disappeared, the quiet transitioned to a shout of "only one Ricky Hatton".
The action within the ring reflected the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, significant bruising developing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the result with scoring tallies of even score, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their inspiration Hatton embodied.
