Former Chelsea academy player Xavier Simons was sent off for Bolton after punching Sheffield Wednesday's Harry Gray during his side's League Cup clash.
The 23-year-old lost his composure in a moment of frustration early in the second half when the match remained goalless.
His sending-off left Bolton a man short for much of the remainder of the contest, and Jamal Lowe capitalised in the 70th minute, netting what turned out to be the only goal of the game.
It was a miserable start to the new campaign for the recently promoted Trotters, exiting the League Cup at the first time of asking.
The flashpoint occurred as Simons attempted to drive forward with the ball, only for Gray, the younger brother of Tottenham's Archie, to haul him back.
Rather than letting the foul go, the Bolton man reacted by swinging his arm towards his opponent, catching Gray in the stomach area.
The Sheffield Wednesday player went down following the impact, and the incident immediately triggered an altercation between players from both sides.
Referee Elliot Bell had little hesitation in producing a straight red card for the midfielder, reducing Bolton to 10 men with more than half an hour still to play.
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Bolton manager Steven Schumacher addressed the incident after the final whistle, expressing his disappointment while choosing not to condemn his player too harshly in public.
He said: "From the angle I've seen it from this side, it looks like Xav has reacted and lashed out, which he knows you can't do.
"It's a poor decision, and it's a decision that you wouldn't expect from him.
"It's the first time I've seen him do that since he's been here, and I'm sure he'll be disappointed with it."
Simons originally came through the ranks at Brentford before moving across west London to join Chelsea's academy, where he spent five years developing as a young player.
He made a single senior appearance for the Blues during the 2021-22 season before heading out on loan spells to Fleetwood Town and Wycombe Wanderers to gain regular first-team experience.
A move to Hull City followed, where he continued to build his reputation in the EFL, before signing for Bolton ahead of last term.
His contribution proved instrumental as the club secured promotion from League One, defeating Stockport County in the play-off final to return to the Championship for the first time since 2019.
Next up for his current club is a home tie against Preston before a trip down to west London to face QPR.
Bolton are one of the favourites to go down this year in the Championship after being promoted back to the second tier last season in the play-offs.
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