The Blues completed a deadline day swoop for the former Atletico Madrid midfielder - signing him on loan with an option to make the deal permanent next summer.
However, Cadena SER says Tuchel only signed the 26-year-old for £4million as a "favour" to his representatives.
He was hooked at half-time during that encounter and didn't feature in their next five Premier League outings before making a short cameo in a 3-0 win at Newcastle on October 30.
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