25-year-old Morata, has started Chelsea's last three matches on the bench and is yet to score for them in 2018 while Cahill failed to appear at all in their second-leg defeat at Barcelona on Wednesday evening.
Chelsea striker Alvaro Morata has seen his form fall off a cliff in recent months, something that has resulted in him being axed at worldwide level.
Spain reached the World Cup by winning nine of their 10 qualifiers, and Lopetegui said the upcoming friendlies are very important in the World Cup preparations.
"[Morata] has had several problems, he has played little but his chances are intact".
La Roja, preparing for the World Cup in Russian Federation this summer, visit Germany on March 23 in Dusseldorf before squaring off against Argentina four days later in Madrid.
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"This was the right time for them to come due to their form and the current situation in the squad", Lopetegui said at a news conference. "He will have an extraordinary end to his season.to make it to the World Cup". We've liked watching him a lot, the reports from his club and the under-21s are excellent and we thought it was a good time see him closer.
He said: "I think we have to try to do both".
"Diego is back with us because he has returned to play but this does not guarantee him a [World Cup] place", Lopetegui said.
Not so long ago, he was considered the first-choice striker for one of the favorites for the World Cup in Russian Federation. Chelsea's Pedro Rodriguez and Victor "Vitolo" Machin were left out.
Atletico striker Costa returns to the national team for the first time since last June, having been left out by coach Julen Lopetegui when the combative forward was unable to play for the club because of its transfer ban.