The 21-year-old is eligible to play for both the country of his birth and England, where he has lived since before his first birthday.
With the tournament fast approaching, Pearce told BBC Radio Nottingham: "At this moment I can't see him going away in January but who knows.
"I think he said to Congo that he would prefer to keep his options open."
An interesting article from September 2013 on the BBC website explains a bit more about Assombalonga's background.