UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight 10-round bout at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a match where the styles never quite clicked, with excessive holding marring the action.
The officials had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They win," the victor said in the ring as he was faced disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, 31, merited the win but the British boxer experienced the second loss of his 28-match career.
The Champion Earns Win in Slow Affair
His career development has plateaued in the past few years through contract problems and extended periods of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He started carefully, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the cleaner periods.
The audience's tolerance decreased during regular grabbing, with Parker going down several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to capture the European light heavyweight championship.
The stadium grew hushed for a extremely emotional homage before the fight as a bell ceremony sounded in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The action within the ring equaled the sentiment. Arthur was knocked down by a significant blow flush on the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured soon, significant bruising forming under his eye as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and courageous, exactly what their inspiration Hatton represented.
