UK light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling single shots in a match where the techniques didn't complement each other, with excessive holding marring the action.
The scoring judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," the victor commented in the ringside as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the decision but the British boxer endured the second setback of his 28-fight career.
The Champion Secures Victory in Dull Affair
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance wore thin because of frequent holding, with Parker going down various instances during close exchanges.
Parker closed well, but many rounds were challenging to assess. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a terrific Manchester vs Manchester battle to secure the European light-heavyweight championship.
The arena grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised match card in the city since Hatton's passing. As the final chime faded, the silence became a shout of "there's just one Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a left hook flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up soon, noticeable damage forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
Arthur edged the verdict with scoring tallies of even score, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their idol the legend embodied.
