UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a contest where the techniques never quite clicked, with frequent clinching disrupting the fight.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi said in the ring as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, merited the decision but the Derbyshire fighter endured the second loss of his 28-match career.
Buatsi Claims Victory in Dull Encounter
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the better stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a terrific local rivalry battle to capture the European light heavyweight championship.
The venue became quiet for a extremely emotional memorial prior to the bout as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the final chime faded, the stillness transitioned to a chant of "only one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the second round, standing uncertainly to his footing on the referee's count.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage appearing under his face as the fighters landed powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the verdict with official scores of draw, close margin and wider gap, but each competitor were determined and full of heart, all the qualities their idol the legend stood for.
