UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, landing some telling single shots in a contest where the techniques never quite clicked, with constant grabbing marring the action.
Two judges scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi stated in the arena as he was faced negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Several boxing experts believed Parker, 31, merited the win but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Uneventful Match
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - maybe in what feels like a last chance - under recent support.
He began cautiously, with the two boxers ready to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to edge the better stanzas.
The spectators' acceptance decreased because of constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester fight to win the continental title.
The venue became quiet for a profoundly moving homage before the fight as 10 bells rang in honor of local hero Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell disappeared, the silence became a roar of "only one Ricky Hatton".
The competition during the bout matched the emotion. Arthur was sent to the canvas by a powerful punch flush on the face in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured soon, noticeable damage appearing under his face as the pair traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of even score, close margin and wider gap, but each competitor were determined and brave, exactly what their idol the legend embodied.
