Sam Allardyce has admitted that Wayne Rooney and Gylfi Sigurdsson cannot both be accommodated in his Everton team because of their lack of pace, raising the prospect that one of the club’s high-profile summer signings will be forced to play a supporting role over the rest of the season.

Rooney was dropped to the bench for the 1-1 draw against West Brom on Saturday, with Sigurdsson moving from the left flank into the No 10 position. In fact Everton toiled despite the change, livening up only briefly when Rooney emerged to play a big part in Oumar Niasse’s undeserved equaliser, but Allardyce says it is hard to envisage a future where both players would start.

“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 [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?’”

Allardyce’s comments can be read, directly or not, as an indictment on a summer transfer policy that led to Everton lavishing a record transfer spend under Ronald Koeman, who preceded Allardyce at Goodison Park. Sigurdsson’s arrival from Swansea cost £40m and the return of Rooney from Manchester United, while on a free transfer, entailed wages of around £150,000 a week. Rooney has had the better season of the pair, scoring 10 goals, although Sigurdsson’s form has picked up under Allardyce and the Iceland international is, at 28, four years younger than his team-mate.

Everton’s poor planning is returning to haunt them and Allardyce, who oversaw a marked improvement in the weeks after his arrival in November, is now deeply concerned about a return of old habits. They remain seven points clear of the relegation zone but the turgid performance against West Brom, added to an abysmal display at Tottenham a week previously, has spooked their manager.

Asked whether he was concerned about slipping back towards the dogfight at the bottom, Allardyce said: “Yes, very, especially after performing like that, and last week’s second-half performance. I’ve seen a huge drain of confidence in the players over the space of one and a half football matches.

“The quality of the Tottenham side, I could accept that, but West Brom are in the bottom three. If anyone should be suffering from nerves it should be them rather than us.”

An even bigger disappointment was the double leg fracture sustained by James McCarthy in denying Salomón Rondón. It happened “in his brave efforts to stop West Brom scoring”, as Allardyce put it; the injury looked ghastly and, although no timescale for McCarthy’s recovery has been laid out, he will surely miss the rest of the season.

McCarthy will hope that, upon his return, Everton remain a Premier League club. “It was a false sense of security if they think like that,” Allardyce said in response to a suggestion that his players had considered themselves safe.

“I know the situation. We’ve gone from top six to bottom four in points collected from two runs of six games. It does happen to everybody apart from the top six, so it’s about me putting it right and about – let’s face it – much better performances than that.”