Manuel Pellegrini warned former club Manchester City of the difficulties they face later in the season in their bid for a quadruple.
City won the Carabao Cup on Sunday and are still in contention for the Premier League, Champions League and FA Cup.
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Their busy schedule may have already started to take its toll as Fernandinho and Aymeric Laporte suffered injuries in the win over Chelsea.
Pellegrini, who led City to a Premier League title in 2013-14, said it would be tough for Pep Guardiola's men to compete for every trophy.
"All is possible in football," he told a news conference as his West Ham side prepare to visit City on Wednesday.
"The way that you continue in all the competitions is more difficult, but I said during the first season that I was in charge that if we continue in the Champions League – we were eliminated by Barcelona – and we continue with the games you must postpone because of the cups, you don't have enough Wednesdays to finish the league.
"Maybe in one week you must play four games, but of course you can do it."
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City's hosting of West Ham will be their seventh game in February as they sit a point behind league leaders Liverpool.
Pellegrini left City in June 2016 and said the club had only made further progress since then.
"They continue to improve every year. Maybe after I finished my contract they brought a lot of new players, invested a lot," he said.
"From the beginning, it was always to be the best team in England with their owners and technical staff, they do it very well."