UK light-heavyweight Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the styles failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi said in the ring as he was faced disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Claims Win in Dull Match
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the cleaner periods.
The audience's tolerance diminished amid constant clinching, with the challenger slipping to the canvas various instances during grappling.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by close verdict in a outstanding local rivalry battle to capture the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage prior to the bout as a bell ceremony tolled in remembrance of local hero Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the final chime ended, the quiet gave way to a roar of "only one Ricky Hatton".
The action within the ring matched the feeling. The eventual winner was sent to the canvas by a significant blow directly to the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were relentless and courageous, all the qualities their inspiration Hatton stood for.
