UK light heavyweight Joshua Buatsi made a winning yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a match where the styles didn't complement each other, with constant grabbing spoiling the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi stated in the ring as he was met by boos from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-bout career.
The Champion Secures Triumph in Dull Match
Buatsi's progress has plateaued in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance wore thin because of constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry battle to secure the European light heavyweight title.
The stadium fell silent for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's death. As the last bell faded, the silence became a shout of "only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up soon, noticeable damage developing under his orbital area as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor edged the result with scoring tallies of draw, 115-113 and clearer margin, but the two athletes were relentless and full of heart, everything their idol the legend embodied.
