British light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful single shots in a match where the approaches failed to mesh, with frequent clinching disrupting the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but what defines champions? They prevail," the victor said in the ringside as he was met by negative reactions from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, 31, deserved the victory but the contender suffered the second loss of his 28-match professional journey.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to take risks in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The audience's tolerance wore thin amid constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light heavyweight belt.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony rang in honor of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound disappeared, the quiet became a shout of "there's just one Hatton".
The action within the ring matched the sentiment. The eventual winner was knocked down by a left hook flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling forming under his orbital area as the fighters exchanged hard punches.
Rea, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were persistent and brave, all the qualities their idol the legend embodied.
