UK light heavyweight Buatsi made a winning yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the approaches didn't complement each other, with excessive holding disrupting the fight.
Two judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They succeed," the victor stated in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-match professional journey.
The Champion Secures Triumph in Slow Affair
His career development has slowed in the past few years through management issues and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter willing to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a solid defense to edge the more effective periods.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas several times during clinches.
The underdog closed well, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry battle to win the continental title.
The venue became quiet for a extremely emotional tribute prior to the bout as ten bells rang in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound ended, the stillness became a roar of "there's only one Hatton".
The fighting during the bout equaled the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was marked up quickly, noticeable damage forming under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of even score, 115-113 and wider gap, but each competitor were persistent and full of heart, everything their hero Hatton represented.
