UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a tight ten-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the styles failed to mesh, with excessive holding disrupting the action.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They win," Buatsi said in the arena as he was faced disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers believed the challenger, thirty-one, should have been awarded the victory but the British boxer endured the second loss of his 28-bout professional journey.
The Champion Claims Triumph in Uneventful Match
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The crowd's patience decreased because of constant clinching, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent all-Manchester fight to secure the European light heavyweight championship.
The stadium grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "there's only one Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was knocked down by a significant blow directly to the face in the initial stage, rising shakily to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising forming under his face as the pair landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
The victor edged the result with scoring tallies of 114-114, 115-113 and clearer margin, but each competitor were determined and full of heart, everything their hero Hatton represented.
