UK light heavyweight Buatsi made a winning yet disappointing debut to his career under new promoters with a decision win 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 bout at the boxing arena.
But the challenger had his moments, delivering effective single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the fight.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi said in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, merited the victory but the British boxer endured the second defeat of his 28-fight career.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has plateaued in the past few years through management issues and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who worked behind a tight guard to barely take the cleaner periods.
The crowd's patience wore thin amid constant clinching, with the challenger slipping to the canvas various instances during grappling.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the continental title.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the location since Hatton's death. As the final chime disappeared, the silence became a roar of "there's just one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was injured soon, significant bruising appearing under his eye as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
The victor edged the decision with judges scorecards of draw, narrow lead and clearer margin, but each competitor were persistent and courageous, all the qualities their hero the legend stood for.
