British light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a fight where the techniques never quite clicked, with frequent clinching disrupting the action.
Two judges scored it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor said in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Various analysts believed the challenger, 31, merited the victory but the British boxer endured the second loss of his 28-fight career.
Buatsi Secures Triumph in Slow Encounter
The fighter's advancement has slowed in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The crowd's patience wore thin because of constant clinching, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. However, former world champion 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 hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester battle to secure the continental championship.
The venue grew hushed for a deeply poignant homage before the fight as 10 bells tolled in memory of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the last bell disappeared, the quiet gave way to a roar of "only one Hatton".
The competition within the ring reflected the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, rising shakily to his footing on the count of eight.
But the red-colored shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising appearing under his orbital area as the pair landed punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the verdict with judges scorecards of draw, narrow lead and clearer margin, but each competitor were relentless and courageous, all the qualities their idol the legend stood for.
