UK light heavyweight Buatsi earned a win yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round contest at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the techniques failed to mesh, with excessive holding disrupting the contest.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the arena as he was met by boos from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-fight career.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience decreased because of frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light-heavyweight title.
The arena grew hushed for a profoundly moving tribute before the fight as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence transitioned to a shout of "there's just one Hatton".
The action during the bout reflected the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his eye as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur barely secured the result with scoring tallies of draw, narrow lead and clearer margin, but each competitor were persistent and full of heart, everything their idol Hatton embodied.
