British light heavyweight Buatsi made a winning yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling individual punches in a match where the styles didn't complement each other, with constant grabbing disrupting the fight.
Two judges had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ringside as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, should have been awarded the victory but the contender endured the second loss of his 28-fight professional journey.
The Champion Secures Win in Slow Affair
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to narrowly win the more effective periods.
The audience's tolerance diminished during constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester battle to secure the continental belt.
The venue became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the city since Hatton's death. As the last bell disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was injured early, noticeable damage appearing under his orbital area as the fighters exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
Arthur barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but the two athletes were determined and courageous, everything their idol the legend stood for.
