UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
The scoring judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They win," the victor stated in the ringside as he was greeted with boos from the spectators.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
The Champion Earns Win in Uneventful Encounter
His career development has stalled in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to barely take the better periods.
The crowd's patience diminished during constant clinching, with the challenger slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but many rounds were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a excellent local rivalry battle to secure the European light heavyweight title.
The stadium became quiet for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the final chime ended, the stillness transitioned to a roar of "there's only one Hatton".
The action during the bout matched the feeling. Arthur was knocked down by a significant blow directly to the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was damaged soon, noticeable damage developing under his face as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the result with judges scorecards of even score, close margin and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration the legend embodied.
