UK light heavyweight Joshua Buatsi earned a win yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round contest at the boxing arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They win," the victor said in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts thought the challenger, thirty-one, deserved the win but the British boxer experienced the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Dull Affair
The fighter's advancement has plateaued in recent years through management issues and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He began cautiously, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the cleaner rounds.
The crowd's patience decreased during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester battle to secure the European light-heavyweight title.
The stadium fell silent for a deeply poignant memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the last bell faded, the stillness became a roar of "there's only one Ricky Hatton".
The competition during the bout matched the emotion. The eventual winner was floored by a significant blow directly to the face in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was injured soon, heavy swelling appearing under his eye as the fighters traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were relentless and courageous, everything their inspiration the legend represented.
