UK light heavyweight Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a fight where the styles failed to mesh, with excessive holding disrupting the action.
The scoring judges judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They succeed," Buatsi said in the ring as he was greeted with boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, deserved the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
Buatsi Claims Win in Uneventful Match
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance wore thin because of constant clinching, with the challenger falling to the floor several times during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight championship.
The stadium became quiet for a profoundly moving memorial before the fight as ten bells rang in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell ended, the silence transitioned to a shout of "only one Hatton".
The fighting within the ring equaled the feeling. Arthur was floored by a significant blow directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the duo traded hard punches.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, narrow lead and clearer margin, but the two athletes were relentless and full of heart, everything their inspiration Hatton stood for.
