UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a competitive 10-round bout at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the techniques didn't complement each other, with frequent clinching disrupting the action.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They win," the victor commented in the ringside as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed the challenger, 31, deserved the victory but the contender endured the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Dull Affair
His career development has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the better stanzas.
The audience's tolerance decreased amid constant clinching, with the challenger slipping to the canvas several times during grappling.
Parker finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester battle to capture the European light-heavyweight belt.
The arena grew hushed for a extremely emotional memorial before the fight as ten bells tolled in memory of local hero Ricky Hatton, who died aged 46 in last month.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell disappeared, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout matched the sentiment. Arthur was knocked down by a powerful punch flush on the face in the early round, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the duo traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of even score, narrow lead and wider gap, but each competitor were determined and brave, exactly what their inspiration Hatton stood for.
