British light heavyweight Buatsi made a winning yet disappointing start to his career under fresh promotional backing 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 tight 10-round fight at the Manchester venue.
But Parker found success, landing some telling single shots in a match where the techniques failed to mesh, with constant grabbing spoiling the fight.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They succeed," the victor stated in the ring as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Several boxing experts felt the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Match
His career development has stalled in the past few years through management issues and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a tentative initial rounds.
The challenger, 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 halfway stage, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to narrowly win the more effective periods.
The spectators' acceptance diminished amid regular grabbing, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry fight to capture the European light heavyweight championship.
The stadium became quiet for a deeply poignant homage before the fight as ten bells rang in memory of local hero Ricky Hatton, who died at age 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the final chime faded, the silence transitioned to a shout of "only one Hatton".
The fighting within the ring matched the feeling. Arthur was floored by a left hook flush on the face in the early round, rising shakily to his feet on the count of eight.
But the traditional red shorts of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was damaged quickly, significant bruising appearing under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur barely secured the verdict with official scores of draw, close margin and clearer margin, but the two athletes were persistent and courageous, everything their inspiration the legend stood for.
