Roberto Mancini dropped Mario Balotelli from the Manchester City squad to face Tottenham last weekend because of "his attitude in training" and the manager has warned the striker it must improve.

Balotelli is yet to score in nine Premier League appearances this season, with Mancini trusting him to start in only five of those. He said: "Mario needs to work harder. It is the same for any player – it is difficult to play at this level if you don't work [in training]. What you do in training will benefit you in the game. Every player needs to work hard and improve his performance in the training session and after he will play well in the game. This is normal. If you want to play well you have to work hard. It is his attitude in training I'm talking about."

Edin Dzeko, Balotelli's fellow forward, also has nine league appearances yet despite making fewer starts – three to the Italian's five – the Bosnian has scored six times. Although Dzeko has stated he is unhappy at not being in the starting XI he continues to make an impact when entering as a substitute. Asked whether Balotelli can learn from this approach, Mancini, whose side host Aston Villa on Saturday, said: "I hope so but Mario is like Edin in that sometimes he can play well, sometimes he can't."

When contacted by the Guardian regarding Mancini's stance, Mino Raiola, Balotelli's agent, said: "I'm not willing to make any declarations at this time. I already said what I have to say [that Mario wants to stay] and I think making declarations about press conferences with Mancini is not helpful at all at this point in time. We respect Roberto – if that's his opinion, he's the manager and we have to respect that." The agent said that the pair's relationship is still healthy.

Balotelli was photographed on a night out on the eve of last Sunday's game with Spurs but Mancini had no problem with this. "No – I told him he was out of the team on Saturday morning and after that he can do whatever he wants," he said. "I took the decision because he didn't work well for three days and I don't like this.

"Mario didn't play because it was my choice. I have 20 players and if there are some players that don't work well, they don't deserve to play. There were other players that worked better than Mario and they deserve to go on the bench. I am a manager, he is a player. If he doesn't work well, he doesn't play. Sometimes he plays well, sometimes not well but he is an important player for us."

During England's 4-2 defeat to Sweden in the midweek friendly Joe Hart received criticism for being at fault for two of Zlatan Ibrahimovic's goals. Both with England and City Hart is the unchallenged No1 and, asked whether the goalkeeper would benefit from competition, Mancini said: "Costel [Pantilimon, Hart's deputy] is a good keeper and if Joe will not play [well] for five, six games, probably I can change the keeper. But I trust totally in Joe.

"I'm sure that also sometime he can some a mistake, this is the keeper's life – I can't change this. For me Joe is the best keeper in England absolutely. He is the first choice but if he [does] not play well I can change, I don't have any problem [doing this]. Joe is like Mario like all the other players. But for England I think it will be the same, if he does not play well maybe the manager can change." Gaël Clichy suffered a back problem in Sunday's 3-2 win over Tottenham Hotspur and Mancini said a decision will be made on Saturday whether he can play against Aston Villa.

Regarding reports that Mancini is interested in signing Luis Suárez, the manager said: "It is not true. We have a good team and we don't need to buy another player in January. We can't buy Suárez or another player because we have four strikers and Suárez plays for Liverpool."