British light heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive ten-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi said in the ringside as he was faced disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Various analysts believed Parker, 31, deserved the win but the contender endured the second defeat of his 28-fight professional journey.
The Champion Earns Victory in Slow Affair
His career development has slowed in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience wore thin because of constant clinching, with the challenger slipping to the canvas several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific local rivalry fight to secure the European light heavyweight championship.
The stadium became quiet for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in remembrance of local hero Hatton, who succumbed at age 46 in recently.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the silence gave way to a shout of "there's just one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was floored by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the eight count.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, noticeable damage forming under his face as the duo exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and wider gap, but the two athletes were persistent and brave, exactly what their inspiration Hatton represented.
