Frank Lampard has just confirmed in his pre-match interview with Amazon Prime Video Sport that Marcos Alonso is injured and that's why Emerson has come in today.
Chelsea face Southampton at Stamford Bridge in Boxing Day.
"In a period with so many fixtures in a short space of time, Callum is a fresh set of legs with pace and it's an opportunity for him to show what he can do," said Lampard.
"At home, we know we haven't been getting the results we want and haven't been clinical in the box at either end. Today is a different challenge to Tottenham in terms of what's expected of us. The fans may not be as loud today, maybe getting over the indulging on Christmas, but hopefully, we can deliver."