UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful single shots in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
Two judges judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They prevail," the victor commented in the ring as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, thirty-one, deserved the win but the British boxer experienced the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has slowed in the past few years through management issues and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry battle to win the European light-heavyweight belt.
The stadium became quiet for a extremely emotional tribute before the fight as ten bells rang in honor of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet became a chant of "there's only one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was knocked down by a powerful punch flush on the chin in the early round, rising shakily to his feet on the referee's count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, heavy swelling appearing under his face as the pair exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and full of heart, all the qualities their idol Hatton stood for.
