British light heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor said in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the win but the British boxer endured the second loss of his 28-bout career.
Buatsi Earns Win in Slow Match
His career development has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors ready to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The audience's tolerance wore thin because of frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker finished the stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Tribute Inspires Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight belt.
The arena became quiet for a profoundly moving memorial before the fight as ten bells tolled in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell faded, the quiet gave way to a roar of "there's only one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the second round, standing uncertainly to his position on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was marked up soon, heavy swelling appearing under his eye as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor barely secured the decision with official scores of even score, 115-113 and wider gap, but each competitor were persistent and full of heart, exactly what their hero Hatton embodied.
