UK light heavyweight Buatsi earned a win yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round bout at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a contest where the techniques never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They prevail," the victor said in the arena as he was met by negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter experienced the second setback of his 28-match career.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the more effective rounds.
The spectators' acceptance wore thin during regular grabbing, with the challenger slipping to the canvas several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Fight
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to secure the European light heavyweight belt.
The venue became quiet for a profoundly moving homage ahead of the contest as 10 bells sounded in honor of local hero Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a roar of "there's only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his eye as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur edged the result with official scores of draw, close margin and 115-112, but the two athletes were persistent and brave, exactly what their inspiration Hatton represented.
