UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker found success, landing some telling clean blows in a contest where the approaches failed to mesh, with constant grabbing disrupting the action.
Two judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my greatest performance but what defines champions? They succeed," the victor stated in the ring as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to earn a 20th career victory.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-bout career.
Buatsi Claims Triumph in Slow Match
His career development has slowed in the past few years through management issues and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to edge the cleaner stanzas.
The audience's tolerance decreased during constant clinching, with the challenger going down various instances during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony rang in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell ended, the silence gave way to a shout of "there's only one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was knocked down by a significant blow flush on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising appearing under his face as the fighters traded powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, narrow lead and clearer margin, but each competitor were relentless and full of heart, all the qualities their hero Hatton embodied.
