Jose Mourinho mentioned before the start of Chelsea's loss to Atletico Madrid that three of his players were nursing injuries -- Eden Hazard, Samuel Eto'o and Oscar. And while Hazard started the match and Eto'o came on in the second half, Oscar never made an appearance, despite being rested for Liverpool.
And now we know why: he wasn't rested against Liverpool at all:
I'm not playing because I felt a pain due to an injury to my hip, so, in the last game against Liverpool, I was not called on. I warmed up [against Atletico], but Mourinho knew that I could hardly play. Maybe I will not play the next game (against Norwich) because I could suffer a more serious injury to my hip and he knows it.
We're aiming to try and play me in the final clash of the Premier League (Cardiff) but I have to take care for the World Cup.
-Oscar. Source: Sky Sports.
Oscar's taken a lot of flack for his form in 2014, perhaps deservedly, but it's an inescapable fact that he's played more football than virtually anyone else his age in the world over the past 24 months, it's probably fairly likely that some combination of fatigue and minor injury has been slowing him down for some time. If he can get a lengthy rest ahead of the World Cup (and a length rest after the World Cup too), we should expect far better out of him next season.
For now, get well soon!