British light heavyweight Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a fight where the approaches never quite clicked, with frequent clinching disrupting the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," the victor stated in the ring as he was greeted with boos from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, deserved the victory but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Claims Victory in Dull Match
His career development has plateaued in recent years through contract problems and considerable time of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who worked behind a tight guard to edge the better rounds.
The audience's tolerance diminished because of constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
Parker closed well, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a excellent local rivalry contest to capture the European light heavyweight title.
The stadium became quiet for a profoundly moving homage prior to the bout as 10 bells sounded in memory of local hero Hatton, who died at age 46 in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime faded, the quiet transitioned to a shout of "there's just one Hatton".
The competition within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured quickly, heavy swelling forming under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of even score, narrow lead and 115-112, but each competitor were relentless and courageous, exactly what their hero Hatton stood for.
