UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a match where the approaches never quite clicked, with excessive holding disrupting the action.
The scoring judges had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor stated in the ring as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed the challenger, thirty-one, merited the win but the contender suffered the second defeat of his 28-bout career.
Buatsi Earns Triumph in Dull Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who operated with a tight guard to edge the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Rea by divided decision in a excellent all-Manchester fight to capture the European light heavyweight belt.
The stadium became quiet for a deeply poignant homage before the fight as 10 bells sounded in memory of local hero Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness gave way to a roar of "only one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his position on the referee's count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was marked up early, noticeable damage appearing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and full of heart, everything their inspiration Hatton stood for.
