UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the action.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They win," the victor stated in the ring as he was faced boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers believed the challenger, thirty-one, deserved the decision but the British boxer experienced the second loss of his 28-fight career.
The Champion Claims Win in Slow Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to barely take the more effective periods.
The crowd's patience decreased during constant clinching, with the challenger going down several times during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester fight to capture the continental championship.
The stadium fell silent for a deeply poignant tribute prior to the bout as 10 bells rang in remembrance of the city's native Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the city since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a chant of "there's only one Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow directly to the jaw in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage forming under his eye as the duo landed powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
The victor barely secured the result with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were persistent and brave, everything their hero Hatton stood for.
