UK light-heavyweight Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a fight where the techniques didn't complement each other, with excessive holding spoiling the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the victory but the contender suffered the second setback of his 28-fight career.
The Champion Claims Win in Uneventful Match
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who operated with a tight guard to narrowly win the more effective stanzas.
The crowd's patience wore thin because of regular grabbing, with the challenger going down multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by divided decision in a terrific local rivalry battle to win the continental title.
The stadium became quiet for a profoundly moving memorial prior to the bout as 10 bells tolled in memory of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the stillness became a roar of "only one Hatton".
The fighting during the bout matched the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the face in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling forming under his face as the fighters exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and wider gap, but both fighters were determined and brave, all the qualities their hero Hatton embodied.
