UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round bout at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the techniques didn't complement each other, with frequent clinching marring the fight.
The officials had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the ring as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts thought the challenger, thirty-one, merited the decision but the British boxer experienced the second loss of his 28-match professional journey.
The Champion Earns Win in Dull Affair
Buatsi's progress has slowed in recent times through contract problems and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss 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 the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who utilized a tight guard to edge the more effective stanzas.
The spectators' acceptance wore thin because of regular grabbing, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the supporting bout, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester contest to capture the European light-heavyweight title.
The venue became quiet for a profoundly moving memorial prior to the bout as 10 bells sounded in memory of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound disappeared, the quiet gave way to a shout of "only one Ricky Hatton".
The competition within the ring equaled the feeling. Arthur was floored by a powerful punch squarely on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured soon, noticeable damage appearing under his orbital area as the pair traded punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the result with official scores of draw, 115-113 and 115-112, but both fighters were relentless and full of heart, exactly what their idol Hatton stood for.
