British light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round bout at the boxing arena.
But the challenger had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but what do champions do? They win," the victor said in the ring as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the win but the British boxer experienced the second setback of his 28-match professional journey.
The Champion Earns Win in Slow Encounter
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a tentative early stages.
Parker, 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 participating in his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner stanzas.
The crowd's patience wore thin because of constant clinching, with Parker going down several times during grappling.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry fight to win the continental title.
The arena fell silent for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in honor of local hero Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the city since Hatton's passing. As the concluding sound faded, the quiet became a shout of "there's only one Hatton".
The fighting during the bout matched the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage developing under his eye as the fighters landed powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the action seldom decreased over the full distance.
The victor narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but each competitor were determined and full of heart, all the qualities their idol Hatton represented.
