UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a contest where the styles failed to mesh, with frequent clinching spoiling the action.
Two judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi said in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought the challenger, thirty-one, should have been awarded the decision but the contender endured the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Dull Match
The fighter's advancement has slowed in recent times through management issues and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The crowd's patience wore thin amid frequent holding, with the challenger going down several times during grappling.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Inspires Local Rivalry Fight
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The venue grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound disappeared, the stillness became a roar of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling appearing under his eye as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, narrow lead and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend embodied.
