British light heavyweight Buatsi earned a win yet unimpressive start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round contest at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the approaches didn't complement each other, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," the victor said in the ring as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
The Champion Claims Victory in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a solid defense to barely take the cleaner periods.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a terrific local rivalry fight to win the European light-heavyweight belt.
The arena fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the final chime faded, the stillness became a roar of "there's only one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in opposing blue - was marked up early, heavy swelling forming under his orbital area as the pair landed punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the result with judges scorecards of draw, close margin and clearer margin, but each competitor were relentless and full of heart, everything their hero the legend represented.
