UK light heavyweight Buatsi earned a win yet unimpressive debut 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 split decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a fight where the techniques didn't complement each other, with excessive holding marring the action.
The scoring judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Several boxing experts felt Parker, 31, merited the victory but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Earns Win in Dull Match
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter ready to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the more effective stanzas.
The audience's tolerance decreased during regular grabbing, with the challenger going down multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a excellent all-Manchester fight to secure the European light heavyweight championship.
The venue grew hushed for a extremely emotional tribute before the fight as ten bells tolled in memory of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell ended, the quiet transitioned to a shout of "only one Hatton".
The competition during the bout equaled the emotion. Arthur was knocked down by a significant blow directly to the chin in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, significant bruising developing under his orbital area as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were relentless and brave, all the qualities their idol Hatton represented.
