Recruitment staff, reports Sky Sports, have been told to scour all available attacking options following Sunday's news that Rashford could be out until March after suffering a double stress fracture in his back.
United boss Ole Gunnar Solskjaer has hinted the club are more likely to look at bringing in a player on a short-term deal.
"It's just an unfortunate situation we're in," Solskjaer told Sky Sports. "It might be, because the window is open, we might look at some short-term deals as well and that could take us through until the summer.
"We're looking at numbers as well because we have had many injuries so if the right one is there, it'll fit for us and we can buy and if there are loans available, it might be possible but I've got players here as well who are chomping at the bit."