ESPN FC Extra: Chelsea-Everton preview
Jose Mourinho has said Oscar is struggling physically, in part due to the "difficult weather" in England.
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Oscar, 22, has made 34 appearances in all competitions for Chelsea this campaign, scoring nine goals.
The Brazil international was in impressive form during the first half of the season, but Mourinho accepts he has not been at his best in recent weeks.
Speaking ahead of Chelsea's Premier League meeting with Everton on Saturday, Mourinho said: "Maybe in the last couple of matches he was not the same, because in the beginning of the season he was the best player in the team.
"He was the player that was really making the difference in the first three, four months of competition.
"In this moment he's not being a match-winner, he's not scoring goals, but he's not playing bad. Not real problems.
"The talent is there all the time, but the physical top condition to express the quality he has is not always there.
"We have to take care of him. He's a young boy. He's not a natural physically strong man, suffers with the accumulation of matches, suffers with the difficult weather we had."
Information from the Press Association was used in this report.