British light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a contest where the techniques didn't complement each other, with excessive holding marring the contest.
The scoring judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor commented in the arena as he was faced disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought Parker, 31, merited the victory but the contender endured the second defeat of his 28-fight career.
Buatsi Earns Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent times through contract problems and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The crowd's patience decreased amid constant clinching, with the challenger going down several times during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight championship.
The arena fell silent for a profoundly moving memorial before the fight as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime faded, the quiet became a chant of "there's just one Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a left hook directly to the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, heavy swelling appearing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
The victor barely secured the result with official scores of even score, 115-113 and clearer margin, but the two athletes were relentless and brave, everything their inspiration the legend represented.
