Latvian striker Rakels suffered a hamstring injury after coming off the bench during the 1-0 defeat at Brighton on Tuesday night.
Brazilian winger Piazon missed that game due to illness and is not expected to have recovered in time to face the Bluebirds.
Midfielder Aaron Tshibola remains sidelined by a hamstring injury but defender Jake Cooper returns after a one-match ban.
Cardiff goalkeeper David Marshall will miss the clash due to a virus, while the visitors are sweating on the fitness of playmaker Peter Whittingham.
Marshall missed last weekend's home victory over Ipswich and Simon Moore will deputise again.
Whittingham felt his hamstring in the second half of the Ipswich game and came off immediately, and Tom Lawrence stands by to replace him.
Fabio Da Silva is available after serving a one-game ban, but Lee Peltier could retain his place at right-back.
MATCH FACTS
- Reading are on a run of 11 games without a win against Cardiff in league competition (D6 L5), last beating their South Wales opponents 5-2 in April 2006.
- Cardiff are unbeaten in six games at the Madejski Stadium (including play-off games), winning two and drawing four.
- The Bluebirds have picked up three wins from their last four matches (L1), winning the last time they were on the road against Bristol City.
- After going five games without defeat, the Royals have lost back-to-back matches for the first time since December.
- Reading have scored more goals in the opening 15 minutes than any other side this season (12).
- Cardiff have conceded a league-high four own goals in the Championship this season.