UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques never quite clicked, with excessive holding marring the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They succeed," the victor commented in the ring as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Several boxing experts thought the challenger, thirty-one, deserved the victory but the British boxer experienced the second setback of his 28-fight professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers willing to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the better stanzas.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester battle to win the continental title.
The arena grew hushed for a extremely emotional homage before the fight as ten bells sounded in memory of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell disappeared, the quiet became a shout of "only one Ricky Hatton".
The action during the bout matched the sentiment. The eventual winner was floored by a significant blow squarely on the face in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising forming under his eye as the pair traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur narrowly won the decision with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were determined and brave, exactly what their idol the legend represented.
