British light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, landing some telling single shots in a fight where the styles failed to mesh, with constant grabbing spoiling the fight.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what defines champions? They win," the victor stated in the arena as he was faced boos from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts believed Parker, 31, merited the win but the British boxer experienced the second setback of his 28-match career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has stalled in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The crowd's patience diminished because of constant clinching, with the challenger going down various instances during grappling.
The underdog ended stronger, but many rounds were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry fight to secure the continental title.
The arena became quiet for a extremely emotional memorial before the fight as ten bells tolled in remembrance of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the silence gave way to a shout of "only one Hatton".
The competition inside the ropes matched the emotion. Arthur was sent to the canvas by a significant blow flush on the jaw in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was marked up quickly, heavy swelling developing under his orbital area as the fighters landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the result with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and brave, everything their hero Hatton stood for.
