UK light-heavyweight Buatsi earned a win yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a match where the approaches didn't complement each other, with frequent clinching disrupting the action.
The scoring judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They succeed," the victor said in the ringside as he was faced negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the win but the contender suffered the second loss of his 28-bout career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to overcommit in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience diminished amid regular grabbing, with Parker slipping to the canvas several times during close exchanges.
The underdog closed well, but several stanzas were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester fight to capture the European light-heavyweight championship.
The arena became quiet for a deeply poignant homage before the fight as ten bells sounded in remembrance of Manchester's own Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the last bell faded, the stillness became a chant of "there's just one Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured soon, significant bruising forming under his orbital area as the fighters traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were determined and full of heart, all the qualities their inspiration Hatton represented.
