UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a match where the techniques didn't complement each other, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the ring as he was faced disapproval from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the win but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
The Champion Earns Win in Dull Match
The fighter's advancement 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 stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner periods.
The spectators' acceptance decreased during constant clinching, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to capture the European light heavyweight championship.
The arena became quiet for a profoundly moving homage before the fight as ten bells rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's only one Hatton".
The competition during the bout equaled the sentiment. Arthur was knocked down by a significant blow flush on the jaw in the second round, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, significant bruising developing under his face as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and courageous, everything their inspiration Hatton stood for.
