British light-heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the techniques failed to mesh, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," Buatsi said in the arena as he was faced boos from the crowd.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the win but the British boxer experienced the second loss of his 28-bout professional journey.
The Champion Earns Win in Slow Match
The fighter's advancement has plateaued in recent years through management issues and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter prepared 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 damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the better periods.
The spectators' acceptance diminished because of frequent holding, with the challenger going down several times during grappling.
The underdog closed well, but many rounds were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to capture the continental title.
The stadium became quiet for a deeply poignant memorial ahead of the contest as 10 bells sounded in memory of local hero Hatton, who died at age 46 in September.
It was the premier televised match card in the city since Hatton's death. As the concluding sound ended, the stillness became a roar of "only one Hatton".
The action within the ring equaled the emotion. The eventual winner was knocked down by a left hook flush on the face in the initial stage, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling developing under his eye as the pair landed powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor edged the result with official scores of 114-114, 115-113 and clearer margin, but each competitor were persistent and brave, exactly what their idol the legend stood for.
