UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the approaches never quite clicked, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the ring as he was faced boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the decision but the British boxer experienced the second setback of his 28-fight career.
The Champion Claims Victory in Uneventful Affair
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, 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 worked behind a solid defense to narrowly win the cleaner stanzas.
The crowd's patience decreased during regular grabbing, with Parker going down various instances during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental belt.
The arena fell silent for a extremely emotional tribute prior to the bout as 10 bells sounded in honor of Manchester's own Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell faded, the quiet transitioned to a shout of "only one Ricky Hatton".
The action during the bout equaled the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the verdict with judges scorecards of 114-114, close margin and wider gap, but each competitor were determined and full of heart, everything their hero the legend embodied.
