British light-heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches didn't complement each other, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what do champions do? They win," Buatsi said in the ring as he was faced disapproval from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the win but the British boxer suffered the second loss of his 28-bout career.
Buatsi Earns Win in Slow Match
Buatsi's progress has slowed in the past few years through contract problems and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the more effective stanzas.
The crowd's patience wore thin during frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester contest to secure the continental belt.
The stadium fell silent for a deeply poignant tribute prior to the bout as a bell ceremony sounded in memory of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the silence gave way to a chant of "only one Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the second round, rising shakily to his footing on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured soon, significant bruising developing under his orbital area as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of draw, narrow lead and 115-112, but both fighters were determined and brave, everything their inspiration Hatton stood for.
