UK light heavyweight Joshua Buatsi made a winning yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close ten-round fight at the boxing arena.
But Parker found success, landing some telling single shots in a contest where the techniques never quite clicked, with frequent clinching spoiling the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi said in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Several boxing experts felt Parker, 31, should have been awarded the victory but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Affair
His career development has stalled in recent years through management issues and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the cleaner periods.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a excellent local rivalry battle to secure the European light heavyweight title.
The venue grew hushed for a deeply poignant memorial before the fight as 10 bells tolled in memory of local hero Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the location since Hatton's death. As the concluding sound ended, the quiet gave way to a chant of "only one Ricky Hatton".
The competition during the bout reflected the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was damaged early, significant bruising appearing under his eye as the duo exchanged punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
The victor edged the result with judges scorecards of 114-114, 115-113 and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero the legend embodied.
