British light heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling clean blows in a contest where the approaches never quite clicked, with frequent clinching spoiling the action.
The scoring judges scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my best showing but what defines champions? They win," the victor commented in the ring as he was faced negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, merited the decision but the contender endured the second loss of his 28-bout career.
The Champion Claims Win in Dull Match
Buatsi's progress has stalled in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to edge the cleaner rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester battle to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional homage ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who died aged 46 in last month.
It was the premier televised fight night in the urban center since Hatton's death. As the last bell ended, the silence transitioned to a shout of "only one Hatton".
The competition within the ring equaled the sentiment. Arthur was knocked down by a left hook flush on the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, noticeable damage appearing under his eye as the pair traded powerful shots.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur barely secured the decision with judges scorecards of draw, narrow lead and 115-112, but both fighters were determined and courageous, all the qualities their inspiration the legend embodied.
