The midfielder suffered a thigh injury during the 3-1 defeat against Stoke City, just minutes after coming off the bench.
Chadli spent time on the sidelines earlier this season with the same problem and he is now facing a further spell on the treatment table.
Pardew is desperate to help the Baggies climb the Premier League table and he admits a replacement will now be at the top of his January wishlist.
He told Sky Sports: “Nacer looks like his injury is as bad as it was originally.
“We think he’s going to miss the first two months of the year, which is a big blow for us. I said after the Stoke game that it was a bigger blow than the result.
“That’s something we’ll have to deal with, something I’m going to have to look at in January because obviously it is a quality player going out of the group for some considerable time.”