British light heavyweight Buatsi made a winning yet underwhelming debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a contest where the styles never quite clicked, with excessive holding disrupting the action.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi said in the ring as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts believed Parker, 31, should have been awarded the victory but the contender endured the second setback of his 28-match professional journey.
Buatsi Earns Win in Slow Match
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a protective stance to edge the cleaner stanzas.
The audience's tolerance decreased because of frequent holding, with Parker going down several times during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to secure the European light-heavyweight championship.
The arena fell silent for a extremely emotional memorial ahead of the contest as ten bells rang in honor of Manchester's own Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell ended, the silence became a roar of "there's just one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was floored by a powerful punch directly to the face in the second round, rising shakily to his position on the eight count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged early, noticeable damage appearing under his eye as the pair landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
The victor barely secured the verdict with official scores of draw, close margin and wider gap, but both fighters were determined and full of heart, everything their hero Hatton embodied.
