Costa is out of Tuesday's Champions League Group G clash with Maribor with a hamstring problem which has plagued him persistently this season
But Mourinho believes the problem was exacerbated while his top-scorer was on duty with Spain, and Costa could now also miss next weekend's match against Manchester United
"It's the same problem that he's having, but now an injury, not an 'almost injury'," Mourinho said
"Let's see what happens for next weekend (at Old Trafford), but next weekend is not important
Important is tomorrow and tomorrow he cannot play."
Costa, scorer of nine goals in seven Barclays Premier League games, missed Saturday's win at Crystal Palace and Chelsea have an unchanged squad for their first fixture with a Slovenian side.
Nigeria midfielder John Obi Mikel (foot) is again absent, while Brazil midfielder Ramires (groin) and World Cup winning Germany forward Andre Schurrle (illness) are also out despite training on Monday.
It is the situation regarding Costa which appears to be frustrating Mourinho most.
"It hurts," added Mourinho, who has not spoken to Del Bosque about Costa.
"You are not happy when you give players in good conditions and you receive the players in bad conditions.
"I had managers with a very open communication, some others they simply don't care.
"Year after year after year, I don't have one single SMS (text message) or one single phone call."
Maribor are unfancied for the match, but coach Ante Simundza believes they can record a surprise result after taking creditable draws against Sporting Lisbon and Schalke in their opening two matches
"Our mentality is to win, to play offensively," Simundza said
"We always try to go for a win, but we will certainly not be pushing very, very hard from the beginning without thinking in this match.
"Chelsea is a very competitive team in every minute, in every second of the game.
"It will be very tough to find some flaws
We will do our best to find it and try to use it.
"Chelsea is the favourite not only for our group but also for the Champions League.
"Every team has strengths and weaknesses and Chelsea is no exception, but Chelsea have more strengths than weaknesses.
"We will try to find these weaknesses to achieve a positive result."
Source : PA