Sam Allardyce believes on-loan midfielder Alex Song will be a "Yaya Toure-type signing" for West Ham this season.

Song swapped the Nou Camp for Upton Park when he moved from Barcelona in August and the 26-year-old has helped lift the Hammers to fourth in the Barclays Premier League table.

West Ham's strong start will be given a rigorous examination when they host defending champions Manchester City on Saturday and when asked about his opponents' world-class stars, Allardyce said: "We've got that type of signing in Alex Song.

"That's a Yaya Toure-type signing for us. He's a top quality player of the highest calibre and he'll provide the sort of service our front line need.

"He links everything together very well so we've got a top quality international player that's played for Arsenal, for Barcelona and in the Champions League.

"He's obviously being slightly overshadowed by Enner Valencia and Sakho at the moment but he'll have a big impact for us."

West Ham played City four times last season and lost on each occasion, conceding 14 goals and scoring just once.

The worst of those defeats came in the Capital One Cup first leg in January when the Hammers were thrashed 6-0 but Allardyce is confident it will be a different story this time around.

"We've got a different team, a different set of players, some of them last season will remember the fact we didn't get anything against City," the West Ham boss said.

"Everyone still talks about the semi-final of the Capital One Cup but there was little we could do with the injuries we had.

"If we're going to challenge against them, it's at this moment in time - City have just come back from playing in the Champions League and we're in good, confident form."

West Ham will be boosted by the returns of James Tomkins and Cheikhou Kouyate.

Defender Tomkins and midfielder Kouyate have recovered from thigh and groin problems respectively while striker Andy Carroll is back in training but not considered ready for the clash at Upton Park.

Guy Demel (thigh), Joey O'Brien (knee) and James Collins (groin) are also out injured.

Former England midfielder Frank Lampard has been ruled out of champions Manchester City's trip.

Lampard suffered a thigh injury in last Saturday's 4-1 win over Tottenham and will not be back in training until next week.

Playmaker Samir Nasri, who had groin surgery last month, has now rejoined the squad and could be back in contention to play next week.

Manchester City boss Manuel Pellegrini believes striker Sergio Aguero has the potential to become the best player in the world.

Aguero's superb start to the season, which has seen him score 11 goals in as many games, including four against Tottenham last weekend, has reignited debate over his standing in the world game.

A year ago comparisons with the likes of Cristiano Ronaldo and Lionel Messi were common but injuries then hampered Aguero throughout the second half of last season.

Those problems, which also affected the Argentina international at the World Cup, now seem behind him and he appears close to his blistering best.

But Pellegrini expects the 26-year-old to get even better and eventually surpass the levels of those currently regarded as the best.

Pellegrini said: "Sergio is a young player, he has a lot of years to improve - three or four years more than other players. It just depends on what he wants to do in the future. I think he has all the conditions to do that.

"I don't think it is the best I have seen from Sergio. I was always sure he would return to his normal performance but I think he can get even better."

City's trip to Upton Park gives them a chance to cut Chelsea's five-point lead at the top of the Barclays Premier League and apply pressure before Jose Mourinho's men face Manchester United on Sunday.

City beat West Ham four times last season, scoring 14 goals, but Pellegrini insists that counts for little and has noted the fourth-placed Hammers' good start this term.

He said: "West Ham are in a good moment, playing good, attractive football.

"They are scoring a lot of goals, they had an important win against Liverpool at home and they have won their last two games.

"I am sure we are going to have a very difficult rival, exactly as we have every game in the Premier League."