British light heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the fight.
The officials judged 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," the victor said in the ringside as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the decision but the contender suffered the second loss of his 28-match career.
The Champion Secures Win in Slow Encounter
Buatsi's progress has stalled in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter willing to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who worked behind a tight guard to narrowly win the more effective rounds.
The crowd's patience diminished amid constant clinching, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester battle to capture the continental championship.
The arena became quiet for a profoundly moving memorial prior to the bout as 10 bells rang in remembrance of Manchester's own Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the concluding sound faded, the stillness became a roar of "there's only one Ricky Hatton".
The action inside the ropes reflected the feeling. Arthur was floored by a significant blow squarely on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored attire of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was damaged quickly, noticeable damage forming under his orbital area as the duo exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur barely secured the decision with official scores of 114-114, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their inspiration Hatton stood for.
