UK light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round fight at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the techniques didn't complement each other, with constant grabbing disrupting the fight.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what do champions do? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought Parker, thirty-one, deserved the win but the contender suffered the second setback of his 28-bout professional journey.
Buatsi Claims Victory in Slow Encounter
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who utilized a protective stance to narrowly win the better stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger going down several times during clinches.
Parker closed well, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent local rivalry fight to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound ended, the silence transitioned to a roar of "only one Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a powerful punch squarely on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was marked up early, noticeable damage forming under his orbital area as the pair exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were persistent and brave, everything their inspiration the legend embodied.
