UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a contest where the styles didn't complement each other, with excessive holding disrupting the action.
Two judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what do champions do? They win," Buatsi commented in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the win but the British boxer experienced the second loss of his 28-fight career.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to barely take the better stanzas.
The crowd's patience wore thin because of regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding local rivalry fight to capture the European light-heavyweight belt.
The arena fell silent for a deeply poignant memorial ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the last bell disappeared, the quiet became a shout of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was floored by a left hook directly to the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged soon, heavy swelling developing under his eye as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor narrowly won the result with scoring tallies of even score, 115-113 and clearer margin, but both fighters were persistent and courageous, everything their idol Hatton stood for.
