UK light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a competitive ten-round fight at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a match where the styles failed to mesh, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi said in the ring as he was faced negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts believed the challenger, thirty-one, deserved the decision but the contender experienced the second loss of his 28-match professional journey.
The Champion Secures Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the better periods.
The audience's tolerance diminished amid constant clinching, with the challenger going down several times during clinches.
The underdog ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry contest to win the European light-heavyweight championship.
The stadium fell silent for a extremely emotional tribute prior to the bout as ten bells tolled in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the final chime ended, the silence transitioned to a chant of "there's just one Hatton".
The fighting within the ring reflected the feeling. Arthur was knocked down by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage developing under his face as the pair exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over 12 rounds.
The victor barely secured the result with scoring tallies of even score, 115-113 and 115-112, but each competitor were relentless and full of heart, all the qualities their idol Hatton stood for.
