UK light-heavyweight Buatsi secured a victory yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a match where the approaches never quite clicked, with excessive holding marring the contest.
The officials had it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the ringside as he was faced disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, merited the victory but the contender experienced the second setback of his 28-match professional journey.
Buatsi Earns Victory in Dull Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a solid defense to edge the more effective stanzas.
The crowd's patience diminished because of regular grabbing, with Parker falling to the floor multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light heavyweight title.
The venue became quiet for a profoundly moving tribute before the fight as 10 bells tolled in honor of the city's native Hatton, who died 46 years old in September.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound faded, the silence became a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured quickly, significant bruising developing under his eye as the fighters exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the verdict with official scores of even score, 115-113 and wider gap, but each competitor were persistent and courageous, everything their idol Hatton embodied.
