The manager was speaking in the aftermath of Saturday's 1-1 draw with West Bromwich Albion in which Oumar Niasse stepped off the bench to earn a largely undeserved point.
Sigurdsson started the match, moving inside to his favoured central role while Rooney watched the first hour from the bench but Everton laboured to create anything until the veteran joined the Icelander on the pitch following James McCarthy's injury.
Rooney helped set up the equaliser with a cross to debutant Theo Walcott whose header was converted by Niasse but Allardyce says that it is unlikely both he and Sigurdsson will start in the same XI.
“Last week I said we have not got a lot of legs in the team and we need to be quicker,” Allardyce said. “We've increased that with Theo 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?'”
Ironically, while he lacks the speed that his new manager is after, Sigurdsson was feted as having covered more ground by March of the 2016-17 season while playing for Swansea City than any other player in the Premier League.
He was eventually prised away from the Swans by Everton for a reported £45m last August, one of three players who prefer the same role that were acquired over the summer, but has battled to live up to his enormous price tag in a struggling Blues team.
Meanwhile, Allardyce has also admitted his concern at Everton's proximity to the bottom three and that his side could get dragged back into a relegation dogfight.
“Especially after performing like that and last week's second-half performance," he said on Saturday.
“I've seen a huge drain in the confidence in the players in the space of one-and-a-half football matches because we didn't do too bad in the first-half against Tottenham.
"The quality of the Tottenham side I can kind of accept but West Brom are in the bottom three and if anybody should've been suffering from nerves it should've been them more than us, but I don't think that was the case today.
“We've gone from top six to bottom four in points in two packs of six games. In the first six we' ve gone right up there and the next six we've got points that equate to relegation so we could get sucked in."