UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, merited the win but the British boxer endured the second setback of his 28-match career.
Buatsi Earns Victory in Slow Affair
His career development has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to edge the cleaner stanzas.
The crowd's patience decreased amid frequent holding, with Parker falling to the floor various instances during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Inspires Local Rivalry Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester contest to capture the European light-heavyweight championship.
The venue became quiet for a profoundly moving memorial before the fight as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a shout of "only one Ricky Hatton".
The action within the ring equaled the feeling. Arthur was floored by a significant blow squarely on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was injured early, significant bruising forming under his face as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor narrowly won the result with judges scorecards of draw, narrow lead and wider gap, but the two athletes were relentless and full of heart, all the qualities their idol Hatton embodied.
