British light heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles failed to mesh, with frequent clinching marring the contest.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They succeed," the victor commented in the ringside as he was faced boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought Parker, 31, merited the victory but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Claims Triumph in Slow Match
His career development has plateaued in the past few years through management issues and considerable time of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under recent support.
He started carefully, with both competitors ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a tight guard to barely take the more effective stanzas.
The spectators' acceptance diminished during constant clinching, with Parker slipping to the canvas several times during grappling.
Parker finished the stronger, but several stanzas were hard to judge. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental belt.
The arena fell silent for a deeply poignant memorial ahead of the contest as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell disappeared, the stillness gave way to a roar of "there's only one Hatton".
The competition during the bout matched the emotion. The eventual winner was floored by a left hook squarely on the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged soon, significant bruising appearing under his face as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the result with scoring tallies of draw, 115-113 and 115-112, but both fighters were determined and brave, exactly what their hero Hatton embodied.
