British light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a contest where the approaches didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," the victor said in the ring as he was greeted with disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Secures Win in Dull Affair
Buatsi's progress has stalled in recent times through management issues and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a calculated opening few rounds.
The challenger, 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 participating in his second bout at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to narrowly win the more effective rounds.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry fight to capture the European light-heavyweight championship.
The venue fell silent for a profoundly moving homage prior to the bout as ten bells tolled in memory of Manchester's own Hatton, who died 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was floored by a left hook squarely on the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, significant bruising appearing under his face as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, exactly what their hero the legend embodied.
