UK light-heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a match where the approaches never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the ringside as he was met by boos from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the win but the British boxer experienced the second defeat of his 28-bout professional journey.
The Champion Earns Win in Dull Affair
His career development has plateaued in the past few years through management issues and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He started carefully, with both competitors ready to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the better periods.
The crowd's patience decreased because of constant clinching, with the challenger slipping to the canvas several times during grappling.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry battle to win the continental belt.
The arena grew hushed for a profoundly moving tribute before the fight as a bell ceremony sounded in remembrance of the city's native Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell faded, the silence transitioned to a shout of "only one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was floored by a powerful punch squarely on the face in the second round, getting up unsteadily to his position on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was injured early, noticeable damage developing under his eye as the fighters landed hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of 114-114, close margin and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton stood for.
