Dragons second row forward Luke Charteris was named in caretaker head coach Robin McBryde's original training squad for the tour, but stayed in Wales to help his region's Heineken Cup play-off ambitions.
After the Dragons secured their Heineken Cup place for the 2009/2010 season with a 42-17 victory over Rugby Calvisano last weekend, Charteris automatically joined McBryde's standby list awaiting the call to international duty.
"We are all disappointed for Bradley who played well against Canada and he is a young man with a big future at this level," said McBryde.
"Hopefully he has learned from the experiences he has had already and he will remain with us in Chicago to help out behind the scenes and continue that learning proccess.
"He is up and about and well in himself and we wish him a speedy recovery as he looks to get back on the pitch in time for the beginning of the season with the Blues.
"Luke has arrived in Chicago, he is a more than able replacement and we are expecting him to slot seamlessly back into the system."