UK light-heavyweight Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a contest where the styles failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Various analysts thought the challenger, 31, merited the victory but the contender endured the second setback of his 28-fight career.
The Champion Secures Triumph in Dull Affair
The fighter's advancement has slowed in the past few years through management issues and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to edge the better rounds.
The crowd's patience decreased amid frequent holding, with the challenger going down several times during clinches.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a excellent local rivalry battle to capture the continental title.
The stadium grew hushed for a extremely emotional memorial before the fight as ten bells tolled in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell ended, the quiet became a shout of "there's only one Hatton".
The fighting inside the ropes matched the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured quickly, noticeable damage forming under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the decision with official scores of even score, narrow lead and wider gap, but each competitor were persistent and courageous, all the qualities their inspiration Hatton embodied.
