British light heavyweight Buatsi secured a victory yet disappointing debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive ten-round fight at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a fight where the styles failed to mesh, with excessive holding marring the contest.
The officials had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, merited the win but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Slow Encounter
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who operated with a solid defense to narrowly win the better periods.
The spectators' acceptance wore thin because of frequent holding, with Parker going down several times during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific local rivalry battle to secure the continental belt.
The arena became quiet for a deeply poignant tribute prior to the bout as ten bells tolled in memory of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the concluding sound ended, the quiet transitioned to a roar of "only one Hatton".
The competition within the ring matched the sentiment. Arthur was floored by a powerful punch squarely on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up early, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were relentless and brave, all the qualities their inspiration Hatton represented.
