British light heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the styles failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ringside as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, merited the victory but the contender experienced the second setback of his 28-bout career.
Buatsi Secures Triumph in Dull Match
His career development has slowed in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a cagey early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance diminished amid constant clinching, with the challenger going down various instances during close exchanges.
The underdog closed well, but many rounds were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial prior to the bout as ten bells sounded in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the first major televised match card in the location since Hatton's death. As the concluding sound disappeared, the quiet became a chant of "only one Hatton".
The competition within the ring matched the emotion. The eventual winner was knocked down by a left hook flush on the chin in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was injured soon, significant bruising developing under his orbital area as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor narrowly won the result with official scores of draw, 115-113 and clearer margin, but both fighters were determined and courageous, exactly what their idol Hatton stood for.
