British light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the boxing arena.
But the challenger found success, connecting with powerful single shots in a match where the techniques failed to mesh, with constant grabbing marring the action.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi commented in the arena as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, merited the win but the contender experienced the second defeat of his 28-match professional journey.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He started carefully, with both competitors ready to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who utilized a solid defense to barely take the more effective rounds.
The crowd's patience wore thin amid regular grabbing, with Parker going down several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Homage Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent local rivalry fight to secure the continental belt.
The arena fell silent for a profoundly moving homage ahead of the contest as 10 bells tolled in remembrance of local hero Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound disappeared, the silence became a shout of "only one Hatton".
The fighting within the ring reflected the feeling. Arthur was knocked down by a left hook directly to the chin in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, noticeable damage appearing under his eye as the fighters landed hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and clearer margin, but both fighters were persistent and courageous, all the qualities their hero Hatton embodied.
