British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi said in the arena as he was greeted with boos from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-match career.
Buatsi Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a tight guard to barely take the more effective stanzas.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the European light-heavyweight belt.
The arena grew hushed for a profoundly moving memorial prior to the bout as ten bells sounded in honor of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the city since Hatton's passing. As the concluding sound faded, the stillness became a shout of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was floored by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured quickly, heavy swelling forming under his orbital area as the pair exchanged punishing blows.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and wider gap, but each competitor were relentless and courageous, all the qualities their hero Hatton represented.
