The star forward did not join his teammates on the pitch at Madrid's Valdebebas training ground on Tuesday morning, joining an injury list of eight first team players who are expected to be unavailable to face City in the high-profile clash at the Bernabéu.
A source told ESPN that Mbappé had a muscular problem and would undergo tests later on Tuesday to determine his availability.
Madrid go into the City game having been beaten 2-0 at home by Celta Vigo in LaLiga on Sunday -- a game in which Mbappé also suffered a broken finger -- with just one win in their last five league games.
ESPN has reported that club executives are now considering replacing Alonso, with the City result a key factor in determining his future.
- Real Madrid's Pérez, board meet amid Alonso future uncertainty - source
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- Xabi Alonso on Real Madrid sack talk: I still have players' support
Mbappé's performances have been a high point for Madrid this season, with the France international scoring 16 goals in LaLiga and nine in the Champions League.
In addition to doubts over Mbappé's participation, Madrid are also currently without defenders Éder Militão, Dean Huijsen, David Alaba, Dani Carvajal, Trent Alexander-Arnold and Ferland Mendy, and midfielder Eduardo Camavinga.
