UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts felt Parker, thirty-one, deserved the win but the contender experienced the second setback of his 28-match professional journey.
The Champion Claims Win in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the better rounds.
The crowd's patience wore thin because of constant clinching, with Parker slipping to the canvas various instances during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry fight to secure the European light-heavyweight title.
The venue became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the final chime ended, the stillness became a roar of "there's only one Ricky Hatton".
The competition during the bout matched the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, noticeable damage developing under his eye as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the result with judges scorecards of draw, 115-113 and clearer margin, but the two athletes were determined and courageous, all the qualities their hero Hatton stood for.
