British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the styles didn't complement each other, with excessive holding marring the contest.
Two judges judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They prevail," Buatsi said in the ring as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Secures Victory in Slow Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner stanzas.
The audience's tolerance wore thin amid constant clinching, with the challenger going down various instances during close exchanges.
Parker closed well, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester fight to capture the European light heavyweight title.
The arena fell silent for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in remembrance of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell disappeared, the stillness gave way to a shout of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was knocked down by a powerful punch directly to the chin in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising forming under his orbital area as the pair landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of 114-114, 115-113 and wider gap, but both fighters were determined and courageous, exactly what their idol the legend represented.
