Marcelo was dribbling past Sanchez when his leading foot went past the ball and caught the Argentine high on the shin with force. The Argentinos player's leg buckled underneath him at a shocking angle, leaving him lying in agony on the pitch.
Sanchez, who was taken to a nearby medical clinic for treatment, suffered a dislocated left knee, Argentinos said in a post on messaging platform X, formerly known as Twitter.
"Today I had a very difficult moment on the pitch," Marcelo posted on Instagram.
"I unintentionally injured a fellow player. I want to wish him the best possible recovery. All the strength in the world."
His Brazilian club also posted a supportive message on social media.
"Fluminense FC expresses its solidarity and wishes a speedy recovery to Argentinos Juniors defender Luciano Sanchez, who was injured in an accidental move in tonight's match," it read.
The first leg of the round of 16 clash ended in a 1-1 draw at the Diego Armando Maradona stadium.
