British light-heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with excessive holding marring the action.
Two judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor stated in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Secures Triumph in Slow Match
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to narrowly win the more effective rounds.
The crowd's patience diminished during constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony rang in memory of local hero Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the concluding sound ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The competition inside the ropes matched the sentiment. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged quickly, heavy swelling appearing under his orbital area as the pair traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the result with official scores of even score, narrow lead and 115-112, but each competitor were persistent and courageous, all the qualities their idol the legend represented.
