El Kun was back to his best yesterday in City's victory over Everton.
The Observer says Real made an inquiry about Agüero in the summer and were told he would not be sold.
Agüero remains their first choice to strengthen the striking options of Carlo Ancelotti, which comprise only Karim Benzema and 20-year-old Alvaro Morata.
Real may firm up their interest for Agüero in the January transfer window.
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