UK light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the techniques never quite clicked, with constant grabbing marring the action.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor stated in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-match professional journey.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has plateaued in recent years through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under recent support.
He began cautiously, with both competitors ready to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner rounds.
The spectators' acceptance decreased because of constant clinching, with the challenger going down various instances during grappling.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the continental belt.
The stadium grew hushed for a deeply poignant tribute before the fight as 10 bells tolled in honor of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the silence became a chant of "there's just one Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged soon, noticeable damage developing under his eye as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over the full distance.
The victor edged the result with scoring tallies of 114-114, close margin and 115-112, but each competitor were relentless and courageous, exactly what their hero Hatton embodied.
