The striker has not trained all week, creating a major headache for the club ahead of the crucial East Midlands clash against rivals Derby County this weekend, who sit in the automatic promotion spots.
It was a position Forest had aspired to earlier in the season but they currently sit in 13th place, 17 points behind Derby.
Pearce said: "Britt is still not on the training pitch at this moment in time because of the knock he received in the game last weekend. I think we'll be missing the likes of Mancienne and we'll see how Fryatt comes out. We'll keep an eye and see how they get on.
"What we need on any given Saturday is everybody available. We'll see how they come through over the next day or so."
Assombalonga has scored 13 of Forest's 33 goals this season, 39 per cent, the third highest ratio any player has contributed to their team in the Championship.
Beating Derby was going to be hard enough without Assombalonga, and missing him would be a huge blow, yet Pearce has not ruled him out entirely and it is possible he could be fit enough at least to feature on the bench.
Pearce has also confirmed the likely absence of Michael Mancienne, who has not played since the December 28 clash with Birmingham, while Matt Fryatt is also one to keep an eye on, as with five goals this season, he could have taken on some of the handed over by Assombalonga.