British light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the approaches didn't complement each other, with excessive holding spoiling the contest.
The officials had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi commented in the arena as he was faced disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt Parker, 31, merited the victory but the contender experienced the second defeat of his 28-fight career.
Buatsi Claims Victory in Slow Encounter
Buatsi's progress has plateaued in recent years through management issues and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who operated with a tight guard to narrowly win the more effective stanzas.
The audience's tolerance diminished because of regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester contest to win the continental belt.
The venue grew hushed for a extremely emotional tribute ahead of the contest as 10 bells rang in honor of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime faded, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout equaled the feeling. Arthur was floored by a left hook directly to the jaw in the early round, rising shakily to his position on the eight count.
But the red-colored attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the duo exchanged powerful shots.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with official scores of even score, close margin and clearer margin, but both fighters were relentless and full of heart, everything their inspiration Hatton represented.
