UK light-heavyweight Buatsi made a winning yet disappointing start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round fight at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a match where the styles failed to mesh, with constant grabbing marring the contest.
Two judges judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but what do champions do? They prevail," the victor stated in the ring as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
The Champion Earns Victory in Dull Affair
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin because of regular grabbing, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light heavyweight title.
The venue fell silent for a extremely emotional homage prior to the bout as a bell ceremony tolled in memory of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell disappeared, the quiet became a chant of "there's only one Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage developing under his face as the duo traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were persistent and full of heart, exactly what their inspiration the legend stood for.
