Costa featured in both of Spain's Euro 2016 qualifiers during the international break, despite needing to manage a hamstring problem since the start of the campaign, and Mourinho has revealed that the forward was back at Stamford Bridge for treatment on Monday morning.
The 51-year-old told reporters: "The Spanish team played in Luxembourg on Sunday. I gave [the Spanish players] two days off so they have to report to train Wednesday at 3pm.
"Diego was here Monday at 9am so you can imagine how strong he was."
The 26-year-old has scored nine goals in as many games in all competitions for the Blues.