UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the styles didn't complement each other, with frequent clinching spoiling the action.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but what do champions do? They succeed," the victor said in the arena as he was faced disapproval from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Earns Victory in Uneventful Affair
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to take risks in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a solid defense to edge the better rounds.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Arthur beat Bradley Rea by divided decision in a outstanding all-Manchester battle to win the European light heavyweight title.
The stadium became quiet for a extremely emotional memorial prior to the bout as ten bells tolled in honor of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the final chime faded, the quiet became a chant of "only one Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, significant bruising forming under his orbital area as the pair landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the verdict with official scores of even score, 115-113 and 115-112, but both fighters were relentless and courageous, everything their idol Hatton stood for.
