Costa suffered a recurrence of his hamstring problem in last month's 3-2 win at Hull and withdrew from international duty with Spain.
Chelsea's top scorer will be in the squad for the Stamford Bridge fixture, but may not start.
Mourinho said: "Diego trained (on Thursday and Friday) with the group
He's not injured
I don't know if I start with him or not.
"We did all the tests and the scans to confirm two days ago scientifically the situation and in this moment the muscle is fine.
"But football is more than that
You need confidence
You don't need to break your intensity
That's our doubt
Let's see next 24 hours."
Asked the long-term solution to Costa's recurring problems, Mourinho insisted an operation would be undertaken only if there was no alternative.
Source : PA
Source: PA