Jose Mourinho has suggested Oscar's decline in form for Chelsea might be due to the player becoming distracted by the prospect of a World Cup in his home country this summer.
Oscar ousted Juan Mata as the Blues' preferred No.10 at the start of the season but has found himself on the periphery in recent weeks, not starting either leg of the Champions League semi-final tie with Atletico Madrid.
Norwich City are 16/1 with Bet365 to beat Chelsea on Sunday
"There are things you can't resolve," the Portuguese admitted. "He was our best player for months, from August to December. He was absolutely fantastic. After that he's been up and down.
"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."
Having also criticised Eden Hazard this week, Mourinho insists all of Chelsea's younger players must stay focused on improving their game if they are to succeed at Stamford Bridge.
"You can't succeed with everybody," he added. "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 Andre Schurrle 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."