British light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a match where the approaches never quite clicked, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor said in the ringside as he was greeted with boos from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts felt the challenger, 31, merited the victory but the British boxer endured the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
His career development has slowed in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a protective stance to barely take the better periods.
The spectators' acceptance wore thin because of frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light-heavyweight belt.
The arena grew hushed for a extremely emotional memorial prior to the bout as ten bells sounded in honor of the city's native Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the urban center since Hatton's death. As the concluding sound ended, the stillness became a shout of "there's just one Hatton".
The action during the bout matched the feeling. The eventual winner was floored by a powerful punch squarely on the chin in the early round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was injured early, heavy swelling forming under his face as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the result with official scores of even score, close margin and 115-112, but each competitor were relentless and full of heart, exactly what their hero the legend stood for.
