It is claimed the Gunners star was on the receiving end of a poor challenge ahead of Arsenal's trip to Liverpool, aggravating Sanchez.
Reports emerged of a bust-up involving Sanchez, which resulted in Arsene Wenger dropping his main man for Saturday's 3-1 Premier League defeat at Anfield.
Sanchez was said to have been confronted by a number of disgruntled team-mates, with the Chile international's long-term future very much up in the air.
Alexis Sanchez started his first game for Arsenal tonight since his training ground bust-up
The 28-year-old appears set to quit Arsenal this summer with just over a year remaining on his contract.
But Gunners coach Pires has played down the seriousness of last week's incident and adds Sanchez was disciplined for his behaviour.
"Claims in the press there was a fight are false – I was there," Pires told beIN Sports.
"There was a bad tackle on AS, a clash, he left the pitch.
Alexis Sanchez loses his cool during Arsenal's clash with Bayern Munich
Tue, March 7, 2017
Alexis Sanchez was not happy during Arsenal's match with Bayern Munich
Marc Atkins / Offside
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Alexis Sanchez rages during Arsenal’s clash with Bayern Munich
"Fans have to understand, there was a disciplinary sanction. Nothing serious happened.
"I was there yesterday too. there is no problem."
Sanchez returned to the Arsenal starting line-up tonight but couldn't help his team overturn their 5-1 deficit against Bayern Munich in the Champions League.
Another 5-1 battering at the Emirates this time saw the Gunners crash out, with Sanchez pictured laughing on the bench after being withdrawn in the second half.