UK light-heavyweight Buatsi made a winning yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful single shots in a match where the approaches failed to mesh, with frequent clinching marring the action.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest performance but how do champions respond? They win," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
The Champion Claims Victory in Dull Affair
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He started carefully, with both competitors prepared to overcommit in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who operated with a tight guard to barely take the cleaner periods.
The audience's tolerance wore thin because of frequent holding, with Parker going down various instances during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent all-Manchester fight to capture the continental title.
The venue became quiet for a deeply poignant homage before the fight as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died aged 46 in last month.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The competition within the ring reflected the sentiment. Arthur was knocked down by a left hook squarely on the face in the early round, standing uncertainly to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, noticeable damage appearing under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor barely secured the result with scoring tallies of even score, 115-113 and wider gap, but the two athletes were determined and brave, all the qualities their idol Hatton stood for.
