British light heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a contest where the techniques didn't complement each other, with constant grabbing marring the action.
The officials scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi commented in the ring as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, thirty-one, deserved the win but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Triumph in Uneventful Affair
Buatsi's progress has stalled in recent times through management issues and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a tight guard to edge the more effective stanzas.
The spectators' acceptance wore thin amid regular grabbing, with the challenger falling to the floor various instances during close exchanges.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Arthur beat Rea by close verdict in a excellent local rivalry contest to capture the European light heavyweight championship.
The venue fell silent for a deeply poignant homage ahead of the contest as 10 bells rang in memory of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the premier televised match card in the urban center since Hatton's death. As the last bell faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The fighting during the bout equaled the feeling. The eventual winner was knocked down by a significant blow flush on the face in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was marked up quickly, noticeable damage developing under his orbital area as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor narrowly won the decision with scoring tallies of even score, close margin and wider gap, but each competitor were relentless and brave, exactly what their hero the legend stood for.
