UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight ten-round contest at the Manchester venue.
But Parker found success, delivering effective individual punches in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
The officials scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They prevail," the victor stated in the ring as he was met by boos from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, merited the decision but the contender suffered the second setback of his 28-fight career.
The Champion Earns Win in Slow Affair
The fighter's advancement has slowed in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner rounds.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor several times during grappling.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The stadium grew hushed for a deeply poignant homage before the fight as 10 bells tolled in memory of the city's native Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the final chime faded, the stillness gave way to a roar of "there's just one Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow directly to the chin in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged quickly, noticeable damage appearing under his orbital area as the duo traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the result with official scores of even score, 115-113 and 115-112, but the two athletes were persistent and courageous, exactly what their hero the legend represented.
