British light-heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close 10-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a match where the approaches didn't complement each other, with frequent clinching marring the action.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi said in the ring as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the contender endured the second loss of his 28-bout career.
Buatsi Claims Triumph in Dull Encounter
Buatsi's progress has plateaued in recent years through contract problems and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to overcommit 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 making just his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to edge the more effective rounds.
The spectators' acceptance decreased during regular grabbing, with the challenger falling to the floor several times during close exchanges.
The underdog closed well, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester contest to secure the European light heavyweight title.
The venue fell silent for a deeply poignant memorial prior to the bout as 10 bells rang in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a shout of "only one Hatton".
The competition within the ring matched the emotion. Arthur was floored by a significant blow squarely on the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising developing under his face as the duo traded hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the decision with judges scorecards of 114-114, close margin and 115-112, but both fighters were relentless and full of heart, exactly what their idol the legend stood for.
