The Harlequins back originally sustained the problem during his side's 24-21 Aviva Premiership victory over Wasps earlier this month and he aggravated the problem further against Leicester the following week.
The 32-year-old will definitely miss Quins clash with Northampton at Franklin's Gardens on Saturday and he could now be set for a sustained period on the sidelines after undergoing a scan on the issue earlier this week.
Brown is now awaiting the results of the scan and Quins director of rugby John Kingston admits he is unsure how long the full-back will be sidelined for.
"Mike is not quite right. He has been away with England for two or three days and they have done some treatment on it," Kingston said.
"After the Leicester game he left in a (protective) boot and it was pretty swollen. On Saturday we'll have a lot more information about whether he could play next weekend."
Should the injury prove a serious one then Brown could miss England's autumn series that is due to get underway against Argentina on November 11 at Twickenham.
Eddie Jones' men are also due to face Australia and Samoa during the three-match series.