Eto'o was stretchered off shortly before half time in Chelsea's 1-0 defeat to Basel, and Mourinho has confirmed that the Cameroonian forward will miss Sunday's match with Southampton after being ruled out for "10 to 15 days".
The 50-year-old told reporters: "Samuel Eto'o is out. The information that we have is that it's a grade one muscular injury which we think is two weeks. It won't be a month, we think around 10 to 15 days."
The 32-year-old has netted four times in 12 appearances since joining Chelsea in the summer.