British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles failed to mesh, with frequent clinching spoiling the action.
Two judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi stated in the ringside as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, merited the victory but the contender suffered the second setback of his 28-fight professional journey.
The Champion Secures Triumph in Dull Encounter
The fighter's advancement has stalled in the past few years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who worked behind a solid defense to edge the better rounds.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Battle
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester battle to win the European light-heavyweight championship.
The stadium fell silent for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the last bell ended, the stillness became a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was injured soon, noticeable damage appearing under his face as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of even score, narrow lead and 115-112, but both fighters were persistent and full of heart, exactly what their hero the legend stood for.
