British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, landing some telling individual punches in a match where the approaches didn't complement each other, with frequent clinching marring the fight.
The scoring judges scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest showing but what do champions do? They succeed," the victor stated in the ring as he was faced boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, merited the victory but the contender endured the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to barely take the better periods.
The audience's tolerance wore thin during constant clinching, with Parker going down various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester contest to secure the European light heavyweight championship.
The arena fell silent for a extremely emotional homage ahead of the contest as ten bells sounded in memory of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound disappeared, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring matched the emotion. The eventual winner was floored by a left hook squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
The victor barely secured the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and full of heart, everything their hero the legend represented.
