UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the action.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor commented in the ringside as he was faced boos from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th career victory.
Various analysts believed Parker, 31, deserved the win but the contender suffered the second setback of his 28-match professional journey.
Buatsi Secures Victory in Uneventful Encounter
His career development has slowed in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance decreased because of constant clinching, with Parker falling to the floor various instances during grappling.
The underdog closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight belt.
The venue became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout matched the feeling. Arthur was knocked down by a significant blow directly to the jaw in the second round, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up early, heavy swelling forming under his face as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were determined and full of heart, exactly what their hero Hatton embodied.
