UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a unanimous decision 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 Manchester venue.
But the challenger had his moments, landing some telling clean blows in a fight where the techniques never quite clicked, with constant grabbing spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They prevail," Buatsi stated in the arena as he was faced boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts thought Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Earns Victory in Uneventful Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to edge the better periods.
The spectators' acceptance decreased because of regular grabbing, with Parker going down several times during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Although, previous titleholder the boxing analyst - 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 likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry fight to capture the European light heavyweight title.
The venue grew hushed for a deeply poignant tribute ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime ended, the stillness became a shout of "there's just one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was knocked down by a left hook directly to the chin in the second round, rising shakily to his position on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his face as the pair landed powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor narrowly won the verdict with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were persistent and brave, exactly what their idol Hatton stood for.
