Federico Fernandez is a very decent player who sometimes gets to play for Argentina, but played a little less often for Napoli after joining them because Walter Mazzarri is kind of nuts. He got loaned to Getafe in the spring and didn't get to play much there either for various reasons. He's 24 and has time to get better, and he's a pretty big guy at 6'2", but his positioning and pace aren't great. He's a nice player and could start for most mid-table Premier League teams, but he's nothing special.
That's the Tottenham you know. The ones that have one of the best defenders in the Premier League, a 21-year-old who has been capped by the England senior team and a borderline club legend coming off his best season in the last three among their central defenders. Oh, and their best defender spent the season injured and is expected to be back for the start of preseason.
Fernandez is a slightly above-average player who would be Tottenham's fifth best central defender. This rumor is stupid.