UK light heavyweight Buatsi earned a win yet disappointing beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the fight.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, deserved the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Claims Victory in Dull Affair
Buatsi's progress has stalled in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the more effective stanzas.
The crowd's patience wore thin amid regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Arthur beat Rea by divided decision in a excellent local rivalry battle to secure the European light heavyweight title.
The arena grew hushed for a extremely emotional tribute ahead of the contest as 10 bells tolled in honor of the city's native Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell disappeared, the stillness became a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was floored by a significant blow flush on the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured early, heavy swelling appearing under his eye as the duo landed punishing blows.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor barely secured the decision with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were determined and courageous, everything their inspiration the legend represented.
