UK light-heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the contest.
Two judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," the victor stated in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Dull Affair
His career development has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to barely take the better rounds.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent local rivalry contest to capture the continental belt.
The venue fell silent for a extremely emotional memorial ahead of the contest as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound ended, the quiet became a shout of "there's just one Hatton".
The competition within the ring equaled the emotion. The eventual winner was floored by a significant blow directly to the jaw in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured soon, noticeable damage appearing under his orbital area as the fighters exchanged punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the verdict with official scores of draw, close margin and clearer margin, but the two athletes were determined and brave, everything their inspiration Hatton embodied.
