UK light heavyweight Buatsi made a winning yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker found success, delivering effective individual punches in a fight where the techniques failed to mesh, with constant grabbing spoiling the action.
Two judges scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest performance but what do champions do? They succeed," the victor said in the ringside as he was met by disapproval from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
Buatsi Claims Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The audience's tolerance decreased because of constant clinching, with the challenger falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to win the European light-heavyweight title.
The arena grew hushed for a extremely emotional memorial before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime disappeared, the quiet became a chant of "only one Hatton".
The competition within the ring matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged soon, noticeable damage forming under his orbital area as the pair landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the result with scoring tallies of draw, narrow lead and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration the legend embodied.
