The New Zealand international had to be taken from the field on a stretcher in the first half of West Ham's 4-1 defeat to Swansea City last weekend after twisting his knee and being knocked unconscious.
Full details of the 29-year-old's injury have not been revealed by the club but Reid outlined the seriousness of the damage in a post on his Instagram.
He said: "Sometimes in life you come across challenges both on and off the field. Lately most of mine have been on with regard to injuries.
"My commitment and love for the game will never stop.
"The way you carry yourself in good times and bad times define you as a person, we have had good and bad days no one is perfect but I assure everyone that I will do everything I can to be back better than before through hard work and dedication.
"Meanwhile we have an important remainder of this season with the team and [it] is important we get the support needed!
"Thank you for all your kind messages! See you next year."