UK light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi stated in the ringside as he was faced boos from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Earns Triumph in Slow Affair
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker going down various instances during grappling.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight title.
The venue became quiet for a extremely emotional homage ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the premier televised match card in the urban center since Hatton's death. As the last bell disappeared, the silence transitioned to a chant of "there's only one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured early, noticeable damage forming under his orbital area as the duo landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of draw, 115-113 and clearer margin, but each competitor were determined and full of heart, all the qualities their idol Hatton stood for.
