UK light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round contest at the boxing arena.
But Parker had his moments, delivering effective single shots in a fight where the approaches didn't complement each other, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They win," the victor commented in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the contender suffered the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
His career development has stalled in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The crowd's patience diminished during frequent holding, with Parker falling to the floor several times during close exchanges.
Parker closed well, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester contest to capture the European light heavyweight belt.
The venue grew hushed for a extremely emotional memorial before the fight as ten bells sounded in honor of local hero Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the emotion. The eventual winner was floored by a powerful punch flush on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured early, heavy swelling appearing under his eye as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were determined and courageous, all the qualities their hero Hatton stood for.
