UK light-heavyweight Buatsi secured a victory yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight 10-round bout at the Manchester venue.
But Parker found success, delivering effective clean blows in a match where the techniques failed to mesh, with constant grabbing marring the fight.
The officials scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They prevail," the victor commented in the arena as he was met by negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts felt the challenger, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-bout career.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to edge the cleaner rounds.
The audience's tolerance diminished amid regular grabbing, with Parker going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Fight
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a excellent local rivalry fight to capture the European light heavyweight championship.
The stadium became quiet for a profoundly moving memorial prior to the bout as 10 bells tolled in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell ended, the silence transitioned to a roar of "there's just one Hatton".
The competition during the bout equaled the sentiment. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was damaged soon, heavy swelling developing under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
Arthur narrowly won the result with official scores of 114-114, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their idol Hatton embodied.
