UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the approaches never quite clicked, with constant grabbing disrupting the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor said in the ring as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the contender suffered the second loss of his 28-fight professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has slowed in recent times through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what seems like a last chance - under recent support.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to barely take the more effective stanzas.
The spectators' acceptance decreased because of constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to secure the European light-heavyweight title.
The venue became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime faded, the quiet transitioned to a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was damaged early, heavy swelling appearing under his face as the fighters traded punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the decision with judges scorecards of draw, close margin and 115-112, but each competitor were relentless and courageous, all the qualities their inspiration the legend embodied.
