British light-heavyweight Buatsi made a winning yet underwhelming start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the approaches failed to mesh, with frequent clinching spoiling the contest.
The scoring judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi stated in the ring as he was faced negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed the challenger, thirty-one, deserved the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has plateaued in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose only previous defeat 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 halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the better periods.
The audience's tolerance wore thin amid constant clinching, with the challenger going down multiple occasions during grappling.
The underdog closed well, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight belt.
The stadium became quiet for a extremely emotional homage ahead of the contest as 10 bells sounded in honor of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised fight night in the city since Hatton's death. As the final chime ended, the silence became a chant of "there's only one Hatton".
The fighting within the ring equaled the emotion. The eventual winner was floored by a left hook directly to the jaw in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was injured early, heavy swelling forming under his eye as the fighters traded powerful shots.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their inspiration Hatton stood for.
