UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut 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 ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the styles didn't complement each other, with frequent clinching spoiling the contest.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor commented in the ringside as he was faced boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the decision but the British boxer experienced the second setback of his 28-fight career.
The Champion Earns Victory in Slow Match
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a solid defense to barely take the more effective rounds.
The spectators' acceptance diminished amid regular grabbing, with Parker going down several times during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to secure the continental belt.
The stadium grew hushed for a profoundly moving tribute before the fight as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised fight night in the city since Hatton's passing. As the final chime faded, the quiet transitioned to a roar of "only one Ricky Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising appearing under his eye as the pair traded powerful shots.
Rea, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were persistent and brave, all the qualities their idol the legend represented.
