UK light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a match where the styles failed to mesh, with frequent clinching marring the contest.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," the victor said in the arena as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent years through contract problems and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to narrowly win the better periods.
The audience's tolerance decreased because of regular grabbing, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Inspires Manchester Derby Fight
In the co-main event, Arthur defeated Rea by split-decision in a terrific all-Manchester fight to capture the European light-heavyweight title.
The stadium grew hushed for a extremely emotional memorial before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime faded, the stillness became a roar of "there's only one Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the jaw in the second round, rising shakily to his footing on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged soon, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
The victor barely secured the result with judges scorecards of even score, narrow lead and 115-112, but both fighters were persistent and courageous, all the qualities their hero the legend embodied.
