Why not? He took the job and said the players were good enough.
To be fair, so did Poch on numerous occasions, then went back to saying we need to rebuild. And the cycle kept repeating. I can see Mourinho doing the same thing if results don't improve.
Mourinho has a big ego, but I don't think he was happy with Aurier at RB and not having a proper DM. He said what he said in order to offer a sense of security. Poch said this for the same reason. Except he grew frustrated and there were times when he couldn't contain himself when talking to the press... Give Mourinho no transfers in January and we might see a different narrative from him.