UK light heavyweight Buatsi earned a win yet disappointing debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a match where the styles never quite clicked, with frequent clinching marring the fight.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the arena as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the win but the British boxer suffered the second setback of his 28-match career.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to edge the cleaner periods.
The spectators' acceptance decreased because of frequent holding, with the challenger going down several times during clinches.
The underdog closed well, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry fight to win the European light heavyweight title.
The stadium became quiet for a extremely emotional tribute ahead of the contest as 10 bells sounded in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured quickly, heavy swelling appearing under his eye as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the decision with official scores of 114-114, close margin and 115-112, but each competitor were determined and full of heart, all the qualities their hero the legend stood for.
