Ole Gunnar Solksjaer is currently without Anthony Martial, while Europa League hero Mason Greenwood is out of action with the flu.
Rashford was named in the United squad to lead the line against West Ham.
But he pulled up with an injury on the hour mark and was forced to make way for Jesse Lingard, who is a winger by trade.
This leaves United, who have been goal shy in recent weeks, without a forward and in a potentially worrying situation.
United now clearly have huge problems upfront, with the sale and loan of Romelu Lukaku and Alexis Sanchez to Inter Milan during the transfer window.