Both players were signed by predecessor Ronald Koeman during the summer and have started 17 and 20 Premier League games respectively since arriving at Goodison Park.
Rooney, now more accustomed to playing in a deeper position just off the striker, was the unlucky player on Saturday as he was benched for the 1-1 draw with West Bromwich Albion.
Allardyce cannot envisage a scenario that sees both the ex-England skipper and Sigurdsson feature in the same XI, admitting that it is too hard to accommodate them in a balanced side.
"Last week I said we have not got a lot of legs in the team and we need to be quicker," he told reporters after the disappointing result. "We've increased that with Theo [Walcott] and Yannick [Bolasie] when he gets back to full fitness but in midfield you have to be able to cover the ground.
"I think Rooney and Gylfi playing together are very shrewd, very clever and talented players but in actual terms of covering the ground it is difficult – it's not their strength.
"So I have to make a big decision on who plays this one and who plays for next one. For me, Gylfi has been trudging away outside on the left side, so play him in the position he wants to play and see what he can do.
"As a team we are light in legs, in pace. And that is something I have to cope with until the end of this season and we look at the whole squad and say: 'What do we do to make it better?'"