UK light-heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the techniques failed to mesh, with constant grabbing disrupting the contest.
Two judges scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor stated in the ringside as he was greeted with disapproval from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, 31, merited the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to take risks in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The crowd's patience diminished because of regular grabbing, with the challenger going down multiple occasions during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a excellent local rivalry fight to secure the European light heavyweight belt.
The venue became quiet for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a roar of "there's only one Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was knocked down by a left hook directly to the chin in the second round, rising shakily to his position on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling forming under his eye as the pair traded hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of 114-114, close margin and clearer margin, but the two athletes were relentless and full of heart, exactly what their idol the legend represented.
