British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the styles never quite clicked, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They succeed," Buatsi said in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the contender suffered the second setback of his 28-fight career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has plateaued in the past few years through management issues and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who worked behind a protective stance to edge the cleaner stanzas.
The crowd's patience decreased during frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry contest to win the European light heavyweight championship.
The arena became quiet for a deeply poignant memorial before the fight as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died aged 46 in last month.
It was the first major televised match card in the city since Hatton's passing. As the last bell faded, the silence transitioned to a chant of "only one Hatton".
The competition during the bout equaled the emotion. The eventual winner was knocked down by a left hook squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured quickly, noticeable damage forming under his orbital area as the fighters traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
Arthur narrowly won the result with judges scorecards of draw, 115-113 and clearer margin, but each competitor were persistent and full of heart, everything their inspiration Hatton stood for.
