British light-heavyweight Buatsi made a winning yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
The officials had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They win," the victor said in the arena as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the win but the British boxer experienced the second defeat of his 28-match career.
Buatsi Earns Victory in Slow Affair
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a tight guard to edge the more effective stanzas.
The crowd's patience diminished because of constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Inspires Manchester Derby Battle
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to win the European light heavyweight championship.
The venue grew hushed for a deeply poignant homage prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who died at age 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell faded, the quiet gave way to a chant of "there's only one Hatton".
The action within the ring matched the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the chin in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage forming under his orbital area as the duo landed hard punches.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of draw, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
