Smith could miss remainder of season
The midfielder had a scan on the joint on Wednesday and complications from a previous injury mean his absence is likely to be far longer than first thought.
Pardew said: "It's sad news for Alan because it's an ankle he's had problems with and we think he'll probably be out for the rest of the season.
"Because he's had a dislocation and a break on that ankle before, when you get a heavy blow like he did it's a very low recovery rate.''
Pardew is also likely to be without striker Andy Carroll for another two or three weeks while Dan Gosling has suffered a setback in his recovery from a serious knee injury.