British light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a contest where the techniques failed to mesh, with constant grabbing marring the contest.
The scoring judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They succeed," the victor said in the ringside as he was met by boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, merited the win but the British boxer suffered the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who utilized a solid defense to edge the better stanzas.
The crowd's patience wore thin amid regular grabbing, with the challenger going down multiple occasions during clinches.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
The victor moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Arthur overcame Rea by divided decision in a excellent all-Manchester battle to win the European light-heavyweight belt.
The stadium became quiet for a deeply poignant homage before the fight as ten bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime faded, the stillness became a chant of "only one Ricky Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up early, heavy swelling appearing under his eye as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were persistent and brave, exactly what their inspiration the legend represented.
