UK light heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques failed to mesh, with constant grabbing disrupting the fight.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the ring as he was faced negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the contender suffered the second setback of his 28-match professional journey.
Buatsi Earns Triumph in Uneventful Match
His career development has stalled in recent times through management issues and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to take risks in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the more effective rounds.
The crowd's patience decreased during frequent holding, with Parker going down several times during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional tribute ahead of the contest as ten bells tolled in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the last bell ended, the silence transitioned to a chant of "there's just one Hatton".
The competition during the bout reflected the feeling. The eventual winner was floored by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured quickly, significant bruising appearing under his eye as the duo traded hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the result with judges scorecards of draw, narrow lead and clearer margin, but each competitor were determined and brave, all the qualities their inspiration the legend stood for.
