UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round contest at the boxing arena.
But the challenger found success, landing some telling clean blows in a match where the approaches never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the ring as he was faced disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Some ringside observers believed the challenger, 31, deserved the victory but the contender experienced the second setback of his 28-match professional journey.
The Champion Secures Win in Dull Match
Buatsi's progress has plateaued in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who utilized a protective stance to edge the cleaner stanzas.
The crowd's patience decreased amid regular grabbing, with Parker slipping to the canvas several times during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester contest to win the continental belt.
The stadium grew hushed for a extremely emotional tribute before the fight as ten bells sounded in honor of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound disappeared, the stillness transitioned to a chant of "there's only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was knocked down by a left hook squarely on the face 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, the opponent - in rival City blue - was marked up early, heavy swelling forming under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of 114-114, narrow lead and clearer margin, but both fighters were determined and courageous, all the qualities their idol the legend stood for.
