José Mourinho will demand improvement next season from his swath of attacking midfielders, principal among them the Brazil international Oscar, as the Portuguese seeks to eradicate some of the inconsistencies that have hampered Chelsea's challenge for prizes.

Oscar will miss the club's final home game of the season on Sunday, against struggling Norwich with a hip injury as Chelsea attempt to maintain their slim chances of securing the Premier League title. Mourinho has seen the form of his first-choice No10 tail off, Oscar scoring in only one league game since New Year's Day, though he expects the 22-year-old to revive for the World Cup.

"There are things you can't resolve," said Mourinho when asked if he was concerned the player was being distracted by the prospect of a major finals in his home country. "He was our best player for months, from August to December. He was absolutely fantastic. After that he's been up and down."

The Chelsea manager went on to suggest that Oscar's toils did not all stem from form and fitness on the pitch. "He was up and down, trying to arrive in a game but not managing it, or being a doubt when the game was too big for a doubtful player. So he arrived in the crucial moment of the season in a low. I can imagine, in a couple of weeks, he will be in a high and during the World Cup he will be in a high."

Mourinho, who expects the club to secure the Spain striker Diego Costa from Atlético Madrid for around £32m this summer, had complained after losing to Crystal Palace at the end of March that many of his younger players lacked the required mentality, but he is convinced he can still coax consistent form from the creative players at the club. "You can help them to build 'them'," he said, referring back to the word "balls" he had scribbled on a journalist's notepad to describe what his players were missing at Selhurst Park. "I'm not saying I'm going to succeed with every one of them.

"But, for example, I have Dr Paco Biosca, the doctor at Shakhtar Donetsk for many years with Willian, and he says he doesn't recognise Willian now: the professional, the player, the commitment. The only thing he recognises is the talent.

"You can't succeed with everybody. You try to help but it's up to them. Every one who is in an age of improvement needs to improve. You can't improve John Terry, Ashley Cole. If you keep them in the position where they are, it's fantastic. All of the others have to improve. Has André Schürrle had a good season? Not bad but next season must be better. Mohamed Salah, Oscar, Willian, Eden Hazard … all of them are in an age of improvement."

Mourinho confirmed that his goalkeeper Petr Cech will undergo surgery on his dislocated shoulder The Portuguese is understood to want Thibaut Courtois to return to the club in the summer after three seasons on loan with Atlético and to compete with Cech for the first-choice position at Stamford Bridge.

The manager awaits news on the one-year contracts offered to Frank Lampard, Terry and Cole. "I did my part on that," he said. "My part was to analyse the squad, analyse the players and advise the club in relation to what I think of the players. That's my job."