British light heavyweight Buatsi made a winning yet disappointing 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 judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They win," the victor stated in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the win but the contender suffered the second loss of his 28-match career.
Buatsi Earns Triumph in Dull Encounter
His career development has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the more effective stanzas.
The crowd's patience decreased during frequent holding, with Parker falling to the floor various instances during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester battle to secure the European light-heavyweight belt.
The stadium became quiet for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the premier televised boxing event in the urban center since Hatton's death. As the last bell faded, the silence became a shout of "there's only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was floored by a left hook flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up quickly, heavy swelling developing under his eye as the duo traded powerful shots.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the result with official scores of 114-114, 115-113 and wider gap, but both fighters were persistent and full of heart, everything their inspiration the legend stood for.
