British light heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round fight at the boxing arena.
But the challenger found success, landing some telling single shots in a fight where the approaches didn't complement each other, with constant grabbing marring the contest.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They win," the victor commented in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, 31, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Claims Victory in Uneventful Affair
Buatsi's progress has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to barely take the better stanzas.
The spectators' acceptance diminished because of frequent holding, with the challenger going down various instances during grappling.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Inspires Local Rivalry Contest
In the secondary fight, Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to secure the continental championship.
The stadium became quiet for a extremely emotional tribute ahead of the contest as a bell ceremony rang in honor of the city's native Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime faded, the stillness gave way to a shout of "there's only one Ricky Hatton".
The fighting within the ring equaled the feeling. Arthur was sent to the canvas by a powerful punch directly to the face in the early round, rising shakily to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising developing under his eye as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
Arthur narrowly won the decision with scoring tallies of draw, 115-113 and 115-112, but the two athletes were determined and brave, all the qualities their hero Hatton represented.
