Rafael Benitez has warned his Liverpool side they cannot make any mistakes in their pursuit of Chelsea and runners-up spot in the Premiership.The Reds are at struggling West Ham tonight and a win could take them to within two points of Chelsea, who do not play until a home match with Blackburn tomorrow. Benitez, who could have Bolo Zenden and Mohamed Sissoko back in his squad after long-term injuries, said: 'If you are chasing Chelsea like we are, we know we cannot make any mistakes. 'We cannot make a mistake at West Ham, we will need those points going into the Everton match on Saturday. 'We must keep going, but with six more points it will be easier for us.'' Liverpool play their first match since the emphatic home win over Chelsea earlier in the month, and Benitez added: 'We have confidence after beating Chelsea, but we must also beat West Ham. We won against a top side and we must beat a side lower down the table. 'In a week when we will have played two games we will know then how near we are to Chelsea. 'How close we are will be clearer. We need three points at West Ham and then another three against Everton, which will be a very difficult game. 'This is a crucial time, we have Manchester United soon and this is a tough period. But the more we win the more confidence we will have.'' The return of Zenden and Sissoko means Liverpool have only Luis Garcia and Harry Kewell on the injured list. Benitez is unlikely to start the game at Upton Park with either of his returning stars, with a place on the substitutes' bench more likely. But the Spaniard is certainly considering throwing Sissoko, now recovered from a dislocated shoulder, into the intense atmosphere of a Mersey derby when Everton visit Anfield on Saturday. He said: 'Momo and Zenden are fit, they are training normally. They may not be match fit, but I would not be worried about playing Momo in the derby. 'Momo has been working fine in training, no problem. He could handle the derby.'' Benitez admits to surprise at seeing West Ham in the relegation zone, saying: 'West Ham will be a tough game. They have a new manager in Alan Curbishley, lots of new players arriving and they must start making an impression and winning matches. 'West Ham players should be more motivated with a new manager. When a team changes a manager, they normally win the first game because everyone wants to impress, then the situation can return to what it was like before. 'This stage of the season is very important for all of us. We have difficult games with Manchester United and Arsenal still to come here, but if we can win some games in a row it will make us closer to the top as the season comes to an end.'