Juanmi has scored seven goals in 24 La Liga matches for Malaga this term and could earn his first cap for the national team.
The addition of Juanmi is sure to raise more concerns about the availability of Costa, who failed to finish Chelsea's 3-2 victory at Hull City on Sunday.
Costa scored the Premier League leaders' second goal at the KC Stadium, but was replaced by eventual match-winner Loic Remy in the 75th minute at the KC Stadium after seemingly struggling with a recurrence of his troublesome hamstring injury.
If the aftermath of the match, Chelsea manager Jose Mourinho said: "I don't know how serious it is. When a striker is playing and you need a goal to win a game with 15 minutes to go, when he says it's over for him, then it's over for me.
"We know his hamstring is not a strong one, he works hard during the week to compensate for this weakness he has there, but an injury can come."
Spain face a crucial Euro 2016 qualifying Group C encounter against Ukraine in Seville on Friday, before visiting Netherlands in a friendly next Tuesday.
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