UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a competitive ten-round bout at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the techniques didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest showing but what defines champions? They succeed," the victor stated in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, merited the victory but the contender endured the second loss of his 28-fight career.
The Champion Secures Victory in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to barely take the more effective stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester contest to capture the European light-heavyweight title.
The stadium grew hushed for a extremely emotional memorial prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his orbital area as the duo landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and brave, everything their idol Hatton embodied.
