British light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But Parker found success, landing some telling clean blows in a match where the styles failed to mesh, with constant grabbing marring the action.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Earns Win in Slow Affair
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The spectators' acceptance wore thin because of frequent holding, with Parker going down various instances during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates Local Rivalry Fight
In the co-main event, Arthur beat Rea by divided decision in a terrific all-Manchester contest to secure the European light heavyweight belt.
The venue fell silent for a deeply poignant tribute ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Hatton, who died aged 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the final chime disappeared, the stillness transitioned to a chant of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage forming under his eye as the fighters landed powerful shots.
Rea, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were determined and brave, everything their hero Hatton represented.
