British light-heavyweight Buatsi earned a win yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close ten-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a contest where the techniques never quite clicked, with frequent clinching marring the action.
The scoring judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest performance but how do champions respond? They win," the victor stated in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed Parker, thirty-one, deserved the victory but the British boxer suffered the second loss of his 28-match career.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who utilized a tight guard to edge the better stanzas.
The crowd's patience decreased during frequent holding, with Parker going down several times during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester fight to win the continental belt.
The arena became quiet for a extremely emotional memorial ahead of the contest as 10 bells tolled in honor of local hero Ricky Hatton, who passed away 46 years old in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, noticeable damage appearing under his face as the pair landed punishing blows.
Rea, 27, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the result with official scores of draw, close margin and wider gap, but the two athletes were determined and full of heart, all the qualities their idol Hatton represented.
