UK light heavyweight Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a match where the approaches failed to mesh, with excessive holding spoiling the action.
The officials had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They win," Buatsi stated in the ringside as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second loss of his 28-fight professional journey.
The Champion Claims Victory in Slow Encounter
His career development has stalled in recent times through management issues and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who operated with a solid defense to edge the better rounds.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light-heavyweight championship.
The arena grew hushed for a extremely emotional memorial ahead of the contest as ten bells rang in honor of the city's native Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell disappeared, the silence gave way to a roar of "there's only one Hatton".
The competition inside the ropes equaled the emotion. Arthur was floored by a left hook flush on the chin in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was damaged quickly, heavy swelling forming under his face as the fighters exchanged punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor narrowly won the result with scoring tallies of 114-114, close margin and 115-112, but the two athletes were determined and full of heart, all the qualities their inspiration the legend represented.
