UK light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my finest display but what do champions do? They prevail," the victor said in the ringside as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Earns Victory in Uneventful Encounter
His career development has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to barely take the cleaner stanzas.
The spectators' acceptance diminished amid frequent holding, with the challenger going down various instances during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional memorial before the fight as a bell ceremony tolled in remembrance of the city's native Hatton, who died at age 46 in recently.
It was the first major televised match card in the city since the passing of Hatton. As the last bell faded, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout matched the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up soon, noticeable damage forming under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the verdict with scoring tallies of draw, close margin and 115-112, but each competitor were persistent and full of heart, exactly what their inspiration the legend represented.
