British light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a fight where the styles never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest display but what do champions do? They succeed," the victor commented in the ringside as he was faced boos from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts felt Parker, 31, merited the victory but the contender suffered the second defeat of his 28-match career.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has slowed in recent years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a solid defense to narrowly win the more effective periods.
The audience's tolerance diminished because of constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Arthur overcame Rea by divided decision in a outstanding local rivalry fight to capture the European light heavyweight title.
The stadium became quiet for a deeply poignant tribute prior to the bout as 10 bells sounded in memory of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound faded, the stillness became a shout of "there's only one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the second round, rising shakily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage developing under his orbital area as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and wider gap, but the two athletes were relentless and full of heart, exactly what their idol Hatton represented.
