British light heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective single shots in a match where the techniques didn't complement each other, with frequent clinching marring the fight.
The officials scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest display but what defines champions? They prevail," the victor said in the ring as he was met by boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, merited the decision but the British boxer endured the second defeat of his 28-bout professional journey.
The Champion Secures Win in Slow Encounter
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He began cautiously, with both competitors ready to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective periods.
The crowd's patience diminished during constant clinching, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester fight to capture the European light heavyweight title.
The arena became quiet for a profoundly moving homage prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the location since Hatton's passing. As the final chime ended, the silence became a roar of "there's only one Hatton".
The fighting within the ring reflected the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up early, noticeable damage developing under his face as the duo landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
Arthur narrowly won the decision with scoring tallies of draw, close margin and 115-112, but each competitor were persistent and brave, exactly what their inspiration the legend represented.
