UK light heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker found success, landing some telling single shots in a match where the approaches failed to mesh, with constant grabbing marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They win," Buatsi said in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed Parker, thirty-one, merited the victory but the contender endured the second setback of his 28-match career.
Buatsi Earns Triumph in Dull Affair
His career development has stalled in recent times through contract problems and considerable time of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under fresh management.
He began cautiously, with the two boxers willing to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who operated with a tight guard to edge the more effective rounds.
The audience's tolerance decreased during frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog 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 a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light heavyweight belt.
The stadium grew hushed for a extremely emotional homage before the fight as 10 bells tolled in remembrance of local hero Hatton, who died aged 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime disappeared, the quiet gave way to a chant of "there's just one Ricky Hatton".
The fighting inside the ropes equaled the emotion. Arthur was floored by a significant blow squarely on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged soon, significant bruising forming under his face as the fighters landed hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor edged the verdict with judges scorecards of draw, narrow lead and wider gap, but each competitor were relentless and full of heart, everything their idol Hatton represented.
