British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the techniques never quite clicked, with constant grabbing marring the fight.
The officials had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They prevail," Buatsi said in the arena as he was met by disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Uneventful Match
The fighter's advancement has plateaued in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter willing to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to edge the cleaner stanzas.
The spectators' acceptance decreased during constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were difficult to score. However, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent local rivalry contest to secure the continental belt.
The arena became quiet for a extremely emotional homage before the fight as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the concluding sound ended, the silence transitioned to a roar of "only one Ricky Hatton".
The competition during the bout equaled the sentiment. The eventual winner was floored by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up soon, noticeable damage forming under his face as the duo exchanged hard punches.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action rarely dropped over the full distance.
The victor edged the decision with judges scorecards of draw, 115-113 and wider gap, but both fighters were relentless and brave, all the qualities their inspiration the legend represented.
