British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight ten-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a fight where the styles failed to mesh, with excessive holding spoiling the contest.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts felt the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Earns Win in Uneventful Match
His career development has slowed in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He began cautiously, with neither fighter ready to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the more effective rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light heavyweight belt.
The arena grew hushed for a extremely emotional homage before the fight as a bell ceremony rang in honor of local hero Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up early, significant bruising developing under his eye as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of draw, narrow lead and wider gap, but the two athletes were persistent and courageous, exactly what their hero the legend stood for.
