British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the approaches failed to mesh, with excessive holding disrupting the contest.
The scoring judges scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor commented in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, 31, deserved the decision but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Earns Triumph in Dull Match
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors ready to overcommit in a calculated early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience diminished because of regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The stadium became quiet for a profoundly moving memorial prior to the bout as 10 bells tolled in remembrance of local hero Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell faded, the stillness became a chant of "there's only one Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, rising shakily to his feet on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up early, heavy swelling forming under his eye as the duo landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of draw, close margin and 115-112, but each competitor were determined and full of heart, everything their hero the legend stood for.
