UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round contest at the boxing arena.
But the challenger found success, delivering effective clean blows in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
Two judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," Buatsi commented in the ringside as he was met by boos 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 twentieth professional win.
Several boxing experts thought Parker, 31, merited the decision but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Earns Triumph in Slow Affair
The fighter's advancement has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who operated with a tight guard to barely take the more effective stanzas.
The crowd's patience wore thin amid constant clinching, with the challenger going down various instances during grappling.
The underdog finished the stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester fight to secure the European light-heavyweight championship.
The venue grew hushed for a extremely emotional homage prior to the bout as a bell ceremony tolled in remembrance of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime disappeared, the stillness gave way to a roar of "only one Ricky Hatton".
The action during the bout matched the emotion. Arthur was floored by a powerful punch squarely on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged quickly, heavy swelling developing under his face as the fighters traded hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
The victor barely secured the result with official scores of draw, close margin and 115-112, but the two athletes were persistent and full of heart, everything their inspiration the legend embodied.
