The Brazilian was hurt in the first half of Wednesday’s 3-0 defeat to Newcastle, which came after he missed United’s three previous matches with an ankle problem.
The 31-year-old, who has not played the full 90 minutes in a match for United since September, could be in a race against time for potentially tough games at the start of December, when United face Newcastle and Chelsea before Bournemouth, Bayern Munich and Liverpool.
His absence means loan signing Sofyan Amrabat could get an extended run as Erik ten Hag’s defensive midfielder.
A statement confirmed: “Casemiro will be out for several weeks due to a hamstring injury suffered during our Carabao Cup defeat to Newcastle United on Wednesday.
“The Brazilian midfielder was withdrawn at half-time due to the issue, and subsequent assessment has revealed a strain that will keep him out for a number of weeks.”