UK light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight ten-round fight at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
The officials scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor commented in the ring as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight career.
Buatsi Earns Win in Slow Encounter
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who utilized a protective stance to edge the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by divided decision in a excellent all-Manchester battle to capture the European light-heavyweight title.
The arena fell silent for a profoundly moving tribute ahead of the contest as a bell ceremony rang in memory of the city's native Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the concluding sound ended, the stillness became a chant of "there's only one Ricky Hatton".
The fighting during the bout equaled the feeling. Arthur was floored by a left hook squarely on the jaw in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, significant bruising forming under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of 114-114, 115-113 and wider gap, but the two athletes were persistent and brave, exactly what their idol Hatton stood for.
