Walcott suffered the injury in training on Thursday night and will fly back to England, where he will be assessed by his club Arsenal.
It is anticipated the winger will be sidelined for between seven and 10 days. It had been anticipated the 24-year-old would start tonight's encounter with San Marino. Instead, he will miss both that game, and Tuesday's crucial trip to Montenegro.
TW14 has been on a barren run of form recently, but with Reading the next opponent there was sure to be fantasy interest in him. However, injured, we may need to look elsewhere. Alex Oxlade-Chamberlain could be an option, but after his goal against Swansea, Gervinho might get the nod.
Of course, Lukas Podolski could come back into the reckoning as well, depending on his own recovery.