UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a fight where the styles didn't complement each other, with constant grabbing marring the action.
The scoring judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi said in the arena as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-match career.
Buatsi Earns Win in Dull Encounter
His career development has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who worked behind a tight guard to edge the better periods.
The crowd's patience wore thin amid constant clinching, with Parker going down various instances during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry battle to capture the European light-heavyweight belt.
The arena became quiet for a extremely emotional memorial ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who died 46 years old in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "there's just one Hatton".
The fighting within the ring reflected the emotion. The eventual winner was knocked down by a left hook squarely on the jaw in the second round, rising shakily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was injured early, significant bruising appearing under his eye as the duo landed hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over twelve rounds.
The victor barely secured the result with judges scorecards of even score, close margin and wider gap, but each competitor were determined and brave, all the qualities their hero the legend stood for.
