Millwall will be without the services of Shaun Williams or Byron Webster for the short trip to Brentford but The Lions boss has backed their replacements to "take their chance" at Griffin Park.
Both the midfielder and defender were ruled out of the Sky Bet Championship fixture after suffering serious knee injuries.
"It's a big blow for both players and for us as a group, especially becuase they have been in outstanding form," Neil Harris told millwallfc.co.uk
"But we have to look ahead now and it's an opportunity for other players. We don't carry a huge squad but we do have plenty of cover.
"To replace Byron there are defenders like Tony Craig, Jake Cooper and Mahlon Romeo. They are three players who have been frustrated at times and have been patiently waiting for an opportunity. I expect them to make the most of that chance, whoever comes in."
Brentford are currently in 19th position, having recorded more draws than any other team in the division.
"I think their positition on the table is a false one," Harris added.
"They have performed really well but have perhaps been a little unlucky in games or haven't had the end product. They have deserved to get more points then they have, they will end up in the middle of the table or upwards.
"It's going to be another challenging game because they are dominant in possession. We will have to make sure we are disciplined and clinical when we get chances.
"It's going to be a great occasion because it's a London derby and it's a great game for my players to be involved in.
"We are really looking forward to it."