UK light heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a fight where the styles failed to mesh, with frequent clinching marring the contest.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor commented in the arena as he was faced disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed the challenger, 31, deserved the victory but the British boxer endured the second loss of his 28-match career.
The Champion Claims Victory in Uneventful Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance diminished during regular grabbing, with the challenger going down several times during grappling.
The underdog closed well, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight belt.
The arena became quiet for a extremely emotional memorial before the fight as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring equaled the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was marked up early, significant bruising developing under his eye as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the result with official scores of 114-114, close margin and wider gap, but both fighters were persistent and courageous, all the qualities their inspiration the legend embodied.
