The forward, who is on-loan from Manchester City, suffered ankle ligament damage during Sunday's 1-0 home defeat to Newcastle and will now return to Eastlands for treatment.
The news will come as a big blow to Bluebirds boss Dave Jones who managed to snare the 21-year-old on a season-long deal. Having put in an impressive substitute appearance at the weekend Etuhu was a strong contender to start at the Madejski Stadium.
Full-back Kevin McNaughton remains the only other absentee, with the 27-year-old a couple of weeks away as he nurses an ankle injury.
Reading defender Darren O'Dea is set to play despite breaking a bone in his hand.
The on-loan Celtic man made his debut in the 0-0 draw with Doncaster at the weekend and picked up the injury but has declared himself fit to keep his place.
Club captain Ivar Ingimarsson could be in the matchday squad for the first time since January after recovering from knee surgery but he is unlikely to return straight to the starting line-up.
Midfielder Brynjar Gunnarsson picked up a knock against Doncaster and could miss out while Republic of Ireland international Shane Long is back in contention after shaking off the slight niggle which forced him to miss out at the weekend.
Grzegorz Rasiak is pushing for his first start after an impressive display from the bench but fellow new signing Jobi McAnuff (back) is still out.
Left-back Chris Armstrong remains sidelined after knee surgery as the Royals search for their first win at home since January.