UK light heavyweight Buatsi earned a win yet underwhelming start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level 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 contest where the styles failed to mesh, with excessive holding disrupting the action.
The officials judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They prevail," Buatsi said in the ringside as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, deserved the win but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Slow Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance decreased amid constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog closed well, but many rounds were difficult to score. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Arthur defeated Rea by split-decision in a excellent all-Manchester fight to secure the continental title.
The venue fell silent for a profoundly moving memorial ahead of the contest as 10 bells sounded in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell disappeared, the silence became a roar of "only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a powerful punch squarely on the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was marked up soon, significant bruising forming under his face as the fighters traded powerful shots.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were persistent and brave, all the qualities their inspiration Hatton embodied.
