Coppell said: "Marcus has a calf problem. As he stepped back to kick the ball he felt something. He has a fairly substantial calf so it must be something fairly serious.
"I would say that he'll be out for a few weeks at least. But (Adam) Federici has been waiting patiently and now it's his chance.
"We'll see what Marcus' condition is and if necessary then we'll have to bring Mikkel (Andersen) back. I'll make Brentford aware of the situation but we can't weaken ourselves just to help someone out."
Andersen only went to Brentford yesterday afternoon.