UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a match where the approaches failed to mesh, with constant grabbing spoiling the action.
The scoring judges scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They succeed," the victor stated in the arena as he was faced negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the win but the contender experienced the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Match
Buatsi's progress has stalled in the past few years through management issues and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to edge the cleaner rounds.
The crowd's patience decreased amid frequent holding, with the challenger slipping to the canvas various instances during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Arthur beat Rea by divided decision in a outstanding all-Manchester fight to win the European light-heavyweight championship.
The stadium became quiet for a profoundly moving memorial before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the city since the passing of Hatton. As the final chime faded, the stillness transitioned to a shout of "only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the chin in the second round, rising shakily to his footing on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the verdict with judges scorecards of draw, close margin and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration the legend represented.
