UK light heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with excessive holding disrupting the contest.
Two judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best display but what do champions do? They succeed," Buatsi stated in the ringside as he was faced negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts thought Parker, 31, deserved the win but the British boxer suffered the second setback of his 28-match career.
The Champion Secures Victory in Slow Affair
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss 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 halfway stage, the underdog had kept it close, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the cleaner rounds.
The crowd's patience wore thin because of frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry battle to capture the continental title.
The arena grew hushed for a profoundly moving homage prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound ended, the silence gave way to a chant of "only one Hatton".
The fighting within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the chin in the second round, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage appearing under his face as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action hardly diminished over the full distance.
The victor edged the result with official scores of 114-114, close margin and clearer margin, but each competitor were persistent and courageous, all the qualities their hero Hatton represented.
