UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a contest where the styles never quite clicked, with frequent clinching marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They win," the victor stated in the ring as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Slow Match
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who utilized a solid defense to edge the more effective periods.
The audience's tolerance diminished because of regular grabbing, with the challenger going down various instances during close exchanges.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight title.
The stadium fell silent for a profoundly moving memorial before the fight as a bell ceremony tolled in honor of the city's native Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the last bell faded, the silence became a roar of "there's just one Hatton".
The action within the ring equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage forming under his eye as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the result with scoring tallies of draw, narrow lead and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration the legend represented.
