UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the techniques never quite clicked, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi commented in the ringside as he was met by disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Various analysts believed Parker, 31, deserved the victory but the contender endured the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter ready to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who utilized a solid defense to narrowly win the more effective periods.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry battle to capture the continental championship.
The arena fell silent for a deeply poignant tribute prior to the bout as a bell ceremony rang in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the concluding sound disappeared, the quiet gave way to a chant of "only one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the face in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured early, noticeable damage developing under his eye as the pair exchanged hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with official scores of draw, close margin and 115-112, but the two athletes were relentless and full of heart, all the qualities their hero Hatton embodied.
