British light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the styles failed to mesh, with constant grabbing marring the contest.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi stated in the ring as he was met by disapproval from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, merited the win but the British boxer endured the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to edge the cleaner rounds.
The audience's tolerance diminished during frequent holding, with the challenger going down various instances during clinches.
Parker closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur overcame Rea by divided decision in a outstanding local rivalry battle to secure the European light heavyweight championship.
The stadium fell silent for a profoundly moving tribute ahead of the contest as 10 bells sounded in honor of Manchester's own Hatton, who died aged 46 in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the stillness transitioned to a chant of "only one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch directly to the face in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, heavy swelling forming under his eye as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur edged the verdict with official scores of draw, 115-113 and wider gap, but each competitor were persistent and brave, all the qualities their hero the legend represented.
