British light-heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a match where the techniques never quite clicked, with excessive holding spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi stated in the ring as he was met by disapproval from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, deserved the victory but the contender endured the second loss of his 28-match career.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to edge the more effective periods.
The crowd's patience wore thin because of constant clinching, with the challenger going down various instances during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester contest to win the European light heavyweight championship.
The arena grew hushed for a extremely emotional memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who died at age 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the last bell disappeared, the silence became a chant of "only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged early, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of even score, close margin and clearer margin, but the two athletes were determined and brave, all the qualities their hero Hatton represented.
