The 22-year-old has gone straight into the Foxes squad for the game against Sheffield United at the Walkers Stadium.
John joined Fulham from Dutch side FC Twente three years ago and has scored 23 goals in 110 appearances for the Cottagers.
However, he has struggled to impress Sanchez and has made only two Premier League appearances from the bench this term - he has not started a match since Boxing Day 2006.