The Belgian forward was signed by the Blues as the long-term replacement for legendary lone striker Didier Drogba.
But he got tired of being a bit-part player under the Portuguese and forced through a loan and then a transfer to Everton.
This week he struck his 50th goal for the Merseyside club, while Chelsea striker Diego Costa is struggling to find the net.
But Courtois, ahead of his side’s Champions League match with Porto, refused to blame the Special One for letting Lukaku move on before he could prove his potential.
When asked if Chelsea and Mourinho made a mistake, Courtois said: “I am really happy for Romelu but we are playing Porto tomorrow [Wednesday] not Everton.”