British light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the approaches failed to mesh, with excessive holding disrupting the action.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They succeed," the victor said in the ringside as he was met by boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts believed Parker, 31, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-match career.
The Champion Earns Triumph in Uneventful Affair
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur beat Bradley Rea by close verdict in a terrific local rivalry contest to capture the continental championship.
The stadium grew hushed for a extremely emotional homage before the fight as a bell ceremony sounded in honor of the city's native Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the stillness gave way to a chant of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was floored by a significant blow directly to the jaw in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was injured soon, noticeable damage developing under his orbital area as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the result with judges scorecards of draw, 115-113 and clearer margin, but both fighters were determined and courageous, everything their inspiration Hatton embodied.
