UK light heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a close ten-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the approaches failed to mesh, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to barely take the better stanzas.
The spectators' acceptance diminished because of constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the supporting bout, Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester fight to secure the continental title.
The arena became quiet for a profoundly moving memorial prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime ended, the stillness became a roar of "only one Ricky Hatton".
The fighting during the bout matched the emotion. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, rising shakily to his position on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, noticeable damage appearing under his eye as the duo landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
The victor narrowly won the decision with official scores of even score, close margin and 115-112, but the two athletes were determined and brave, all the qualities their idol Hatton stood for.
