British light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling clean blows in a contest where the approaches never quite clicked, with constant grabbing marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They prevail," the victor commented in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the win but the contender experienced the second defeat of his 28-bout career.
The Champion Earns Victory in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a solid defense to narrowly win the more effective rounds.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Homage Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry contest to capture the European light heavyweight belt.
The arena became quiet for a deeply poignant tribute before the fight as 10 bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the last bell faded, the stillness gave way to a roar of "only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, rising shakily to his feet on the eight count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, significant bruising developing under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over twelve rounds.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration Hatton represented.
