UK light heavyweight Buatsi secured a victory yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round bout at the boxing arena.
But the challenger found success, landing some telling single shots in a contest where the techniques never quite clicked, with excessive holding marring the fight.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but what defines champions? They succeed," the victor commented in the ring as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Triumph in Slow Encounter
Buatsi's progress has stalled in recent years through contract problems and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The spectators' acceptance decreased during regular grabbing, with Parker going down several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Arthur defeated Rea by divided decision in a terrific all-Manchester battle to win the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage prior to the bout as 10 bells rang in memory of Manchester's own Hatton, who died 46 years old in last month.
It was the premier televised boxing event in the urban center since Hatton's passing. As the final chime disappeared, the stillness transitioned to a chant of "only one Ricky Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up soon, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the result with official scores of 114-114, close margin and 115-112, but the two athletes were relentless and full of heart, all the qualities their hero Hatton represented.
