UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a fight where the styles failed to mesh, with excessive holding marring the contest.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest showing but what defines champions? They win," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-match career.
Buatsi Secures Victory in Dull Match
His career development has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who operated with a solid defense to edge the better rounds.
The crowd's patience decreased because of regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to secure the continental championship.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in memory of Manchester's own Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the concluding sound ended, the silence became a shout of "only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling developing under his face as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but both fighters were determined and brave, everything their idol the legend stood for.
