British light-heavyweight Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a match where the styles didn't complement each other, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ringside as he was faced negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Various analysts thought Parker, thirty-one, merited the win but the contender suffered the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to take risks in a calculated initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the better rounds.
The spectators' acceptance decreased during constant clinching, with the challenger slipping to the canvas various instances during grappling.
Parker closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester battle to secure the European light heavyweight championship.
The arena became quiet for a deeply poignant memorial before the fight as ten bells rang in remembrance of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound faded, the quiet became a roar of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a left hook flush on the jaw in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling appearing under his eye as the pair traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with scoring tallies of even score, narrow lead and 115-112, but the two athletes were persistent and brave, everything their hero the legend stood for.
