UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life 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 ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the contest.
Two judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but what do champions do? They win," Buatsi stated in the ring as he was met by boos from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts believed Parker, 31, merited the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to edge the more effective periods.
The crowd's patience wore thin during constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog closed well, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by divided decision in a terrific local rivalry contest to win the European light heavyweight championship.
The arena became quiet for a extremely emotional homage before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the concluding sound disappeared, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action inside the ropes equaled the emotion. The eventual winner was floored by a left hook flush on the chin in the second round, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged early, noticeable damage appearing under his face as the pair exchanged powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and wider gap, but each competitor were persistent and courageous, all the qualities their idol Hatton stood for.
