Roberto Mancini has warned that Manchester City's chances of success in the Premier League and Champions League next season depend on not repeating the mistakes of last summer's recruitment drive. He believes it is "impossible" to sell players before buying, as was the policy, with Mancini pointing to Bayern Munich, who after securing the Bundesliga have agreed to purchase Borussia Dortmund's Mario Götze.

In the last close season Mancini missed out on his five first-choice targets of Robin van Persie, Eden Hazard, Javi Martínez, Daniele De Rossi and Daniel Agger, and was forced to settle for Jack Rodwell, Scott Sinclair, Matija Nastasic, Javi García and Maicon.

Nastasic apart, all have struggled to establish themselves in the Italian's team, and Mancini urged the City hierarchy, which is led by Ferran Soriano, the chief executive, and Txiki Begiristain, the director of football, to act swiftly this summer.

"It's important that we work well," said Mancini. "From now to August 30 [the close of the transfer window], we [must] work well and not like last year. Maybe we can do better because for two years we worked well, we had our targets and we got our targets. Three years ago we wanted to get to the Champions League and we got the Champions League and won the FA Cup. Last year we wanted to fight for the title and we won the title. This year we didn't work well but this hasn't been a bad season and, if we win the FA Cup, it is a good season. It's not important only to work well on the pitch, it's important you start to work well."

Mancini, asked if securing his prime targets was the crucial factor for success next season, said: "Yes. It's important not to make a mistake on new players. We need to do everything well."

Has the board learned from last summer? "We are people and we can make mistakes," said the manager. "No one wants to make mistakes but it can happen. It's important that we know last year we didn't work well and I hope that this year we can improve this. We have a good relationship for this. We've talked about the squad all the time that we can talk about the future, the players, but it's important that we work well."

Regarding the previous strategy of selling before buying, Mancini said: "This is impossible. Like you saw in the last week at Bayern Munich [taking Götze] – the best team in this moment with Borussia. I've read that Barça need six players – Barça! We need to add to our team."

Bayern have criticised City for their lavish-spending approach to building a successful club. Yet despite Bayern agreeing to pay the £32m buy-out clause to acquire Götze and their interest in taking another of Dortmund's best players, Robert Lewandoski, Mancini refused to return this criticism. "It's not important. In football it's good because all the people asked how it's possible that City don't win the title after spending a lot of money," he said. "There are other teams that spend more than us but this is football."

Mancini also praised the approach of Manchester United in taking the title from City. "They have a better attitude," said the manager. "We need to improve our attitude in some games and continue to work because in the end if we get second and we're in the FA Cup final, that's not a bad season."

For Saturday's visit of West Ham United Mancini expects a stern challenge. "It will be difficult, they are a physical team. [Andy] Carroll is a strong striker," he said.