British light heavyweight Buatsi earned a win 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 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles never quite clicked, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what do champions do? They win," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-fight career.
The Champion Secures Triumph in Uneventful Affair
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance diminished because of frequent holding, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving memorial before the fight as a bell ceremony rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a significant blow directly to the chin in the initial stage, rising shakily to his position on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged soon, heavy swelling forming under his orbital area as the duo exchanged punishing blows.
Rea, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with official scores of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, all the qualities their inspiration Hatton stood for.
