British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a match where the styles never quite clicked, with frequent clinching marring the fight.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best display but how do champions respond? They succeed," Buatsi said in the ringside as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Several boxing experts felt the challenger, 31, merited the decision but the Derbyshire fighter endured the second setback of his 28-bout professional journey.
Buatsi Claims Victory in Dull Match
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The audience's tolerance diminished during frequent holding, with the challenger going down several times during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester contest to win the European light-heavyweight championship.
The arena grew hushed for a profoundly moving homage before the fight as 10 bells rang in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the concluding sound disappeared, the quiet gave way to a shout of "only one Hatton".
The competition inside the ropes matched the sentiment. Arthur was knocked down by a powerful punch directly to the jaw in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling developing under his eye as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the decision with official scores of draw, 115-113 and clearer margin, but the two athletes were determined and courageous, exactly what their inspiration the legend stood for.
