UK light-heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a match where the styles failed to mesh, with frequent clinching disrupting the action.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi stated in the ring as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts believed the challenger, 31, deserved the victory but the British boxer suffered the second setback of his 28-bout career.
Buatsi Earns Win in Slow Match
His career development has slowed in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the better stanzas.
The spectators' acceptance diminished during frequent holding, with Parker going down multiple occasions during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a terrific local rivalry battle to secure the European light-heavyweight belt.
The stadium fell silent for a profoundly moving tribute before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "there's just one Ricky Hatton".
The action within the ring reflected the feeling. The eventual winner was sent to the canvas by a left hook squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up quickly, heavy swelling appearing under his eye as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor edged the result with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were persistent and full of heart, everything their idol Hatton stood for.
