Spain forward Diego Cost is likely to wear the No.18 jersey for his old/new club.
After spending three seasons at Chelsea, having helped the Blues win two Premier League titles, the Brazilian is back at his old stomping ground - although not in a literal sense as the Rojiblancos recently left the Vicente Calderon for their new Wanda Metropolitano home.
The striker won't be able to play for the La Liga side until January, as they're serving a transfer ban, but the club have hinted that he could be assigned the No.18.
The number is quite popular with strikers, and it is very likely to be Costa's, with Fernando Torres wearing the No.