UK light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive 10-round contest at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the styles failed to mesh, with frequent clinching spoiling the fight.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They succeed," Buatsi said in the ring as he was greeted with disapproval from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the decision but the British boxer endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Slow Match
Buatsi's progress has stalled in the past few years through management issues and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who operated with a solid defense to narrowly win the better periods.
The spectators' acceptance decreased amid frequent holding, with Parker going down various instances during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester battle to win the continental title.
The arena grew hushed for a extremely emotional memorial before the fight as a bell ceremony tolled in memory of local hero Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the silence became a chant of "there's only one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was floored by a powerful punch flush on the face in the second round, rising shakily to his footing on the count of eight.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising developing under his face as the pair exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with scoring tallies of draw, 115-113 and 115-112, but each competitor were persistent and courageous, everything their inspiration Hatton embodied.
