British light heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a tight ten-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the fight.
Two judges scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi stated in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the decision but the contender experienced the second defeat of his 28-fight professional journey.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has slowed in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who utilized a tight guard to barely take the better periods.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to win the European light heavyweight belt.
The venue fell silent for a profoundly moving homage ahead of the contest as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime ended, the silence became a chant of "there's just one Hatton".
The action within the ring reflected the feeling. Arthur was sent to the canvas by a left hook flush on the face in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was injured early, significant bruising developing under his face as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with scoring tallies of even score, 115-113 and 115-112, but the two athletes were determined and courageous, everything their hero Hatton stood for.
