UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches failed to mesh, with constant grabbing spoiling the action.
Two judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They win," the victor stated in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Earns Win in Uneventful Affair
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner stanzas.
The crowd's patience diminished because of regular grabbing, with Parker going down various instances during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester fight to secure the European light-heavyweight belt.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells sounded in honor of the city's native Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the silence gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was floored by a left hook squarely on the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling developing under his eye as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of 114-114, close margin and 115-112, but both fighters were relentless and brave, exactly what their inspiration the legend stood for.
