UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing disrupting the action.
Two judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They prevail," Buatsi commented in the ringside as he was faced disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed the challenger, thirty-one, should have been awarded the decision but the British boxer endured the second setback of his 28-fight career.
The Champion Earns Triumph in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective rounds.
The crowd's patience wore thin amid regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester fight to win the European light-heavyweight title.
The stadium grew hushed for a deeply poignant tribute prior to the bout as ten bells tolled in honor of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound faded, the quiet became a shout of "there's just one Ricky Hatton".
The fighting during the bout matched the feeling. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the pair traded hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the verdict with official scores of 114-114, close margin and wider gap, but both fighters were persistent and brave, everything their idol the legend embodied.
