British light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a fight where the techniques failed to mesh, with constant grabbing marring the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what do champions do? They win," the victor commented in the ring as he was faced boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed the challenger, thirty-one, merited the win but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Claims Victory in Slow Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to edge the more effective stanzas.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor several times during grappling.
The underdog ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry contest to secure the European light heavyweight title.
The venue fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in memory of local hero Hatton, who succumbed 46 years old in September.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound faded, the stillness became a chant of "only one Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the chin in the second round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling appearing under his face as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the decision with official scores of 114-114, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their inspiration the legend represented.
