UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close 10-round contest at the Manchester venue.
But the challenger found success, landing some telling individual punches in a contest where the techniques failed to mesh, with constant grabbing disrupting the action.
The officials scored it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They win," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-bout career.
The Champion Secures Victory in Dull Affair
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a calculated initial 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 participating in his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the better stanzas.
The spectators' acceptance decreased because of constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker closed well, but several stanzas were hard to judge. However, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light heavyweight belt.
The venue fell silent for a extremely emotional homage ahead of the contest as a bell ceremony rang in remembrance of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell ended, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The action within the ring reflected the feeling. Arthur was knocked down by a significant blow squarely on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged soon, heavy swelling appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with official scores of even score, close margin and 115-112, but the two athletes were persistent and courageous, everything their hero Hatton represented.
