British light heavyweight Buatsi made a winning yet disappointing debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive ten-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a fight where the techniques failed to mesh, with constant grabbing marring the contest.
Two judges judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi stated in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed the challenger, 31, deserved the victory but the contender endured the second loss of his 28-bout career.
Buatsi Claims Victory in Slow Encounter
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester fight to secure the continental championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells rang in honor of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the stillness became a roar of "there's just one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the early round, getting up unsteadily to his position on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage developing under his face as the pair exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor edged the result with scoring tallies of even score, close margin and wider gap, but each competitor were persistent and courageous, exactly what their idol the legend embodied.
