UK light-heavyweight Buatsi made a winning yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the styles failed to mesh, with frequent clinching spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," the victor stated in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has stalled in recent years through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with both competitors prepared to take risks in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific local rivalry battle to secure the European light-heavyweight title.
The stadium became quiet for a profoundly moving tribute prior to the bout as ten bells rang in remembrance of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound faded, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. The eventual winner was knocked down by a left hook squarely on the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage forming under his orbital area as the duo landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity rarely dropped over the full distance.
The victor edged the decision with judges scorecards of even score, close margin and wider gap, but each competitor were determined and brave, exactly what their inspiration the legend stood for.
