British light-heavyweight Joshua Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a match where the styles didn't complement each other, with frequent clinching marring the action.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They win," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the win but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who operated with a solid defense to edge the more effective rounds.
The spectators' acceptance decreased amid constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental belt.
The venue became quiet for a deeply poignant homage prior to the bout as a bell ceremony rang in honor of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was floored by a powerful punch flush on the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured early, noticeable damage appearing under his orbital area as the duo landed powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over the full distance.
The victor narrowly won the decision with official scores of draw, narrow lead and clearer margin, but the two athletes were persistent and brave, all the qualities their hero Hatton represented.
