UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful individual punches in a match where the styles failed to mesh, with constant grabbing disrupting the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the ring as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the British boxer endured the second defeat of his 28-bout career.
Buatsi Secures Win in Uneventful Affair
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to edge the more effective rounds.
The spectators' acceptance wore thin during frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to win the continental championship.
The arena fell silent for a profoundly moving homage ahead of the contest as ten bells tolled in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound faded, the quiet gave way to a shout of "there's only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a left hook flush on the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, heavy swelling appearing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor barely secured the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their idol Hatton stood for.
