British light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques never quite clicked, with constant grabbing spoiling the action.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They succeed," Buatsi stated in the arena as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought the challenger, 31, deserved the decision but the British boxer endured the second setback of his 28-bout career.
The Champion Earns Win in Uneventful Encounter
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a cagey 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 appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The spectators' acceptance diminished amid constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent all-Manchester contest to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional tribute before the fight as ten bells tolled in memory of Manchester's own Hatton, who passed away at age 46 in last month.
It was the first major televised match card in the city since Hatton's death. As the last bell ended, the silence became a roar of "there's only one Ricky Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the face in the early round, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged soon, heavy swelling forming under his face as the pair traded punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the decision with official scores of even score, 115-113 and clearer margin, but each competitor were relentless and courageous, exactly what their inspiration Hatton embodied.
