British light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the fight.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the arena as he was faced negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Slow Affair
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to barely take the more effective rounds.
The audience's tolerance diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Inspires Local Rivalry Battle
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester fight to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony sounded in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised fight night in the urban center since Hatton's passing. As the final chime faded, the quiet became a shout of "only one Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged early, heavy swelling appearing under his orbital area as the duo traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, everything their hero Hatton embodied.
