The striker was in some pain at the end of the Arsenal game in midweek.
"The good news is he hasn't fractured anything," reported Lampard. "We were worried about a small fracture or a hairline fracture. He hasn't got that. We have to see how it develops. It's not a clear-cut one.
"He's not fit for this weekend. Whether he will be fit for Leicester after that I'm still not sure. I haven't got the news for you. It's not clear at the moment."
With a number of leading English attackers currently out injured, Lampard considered if the effects of the busy Christmas period were being felt.
"There is no absolute science for that. Maybe with muscle injuries you can link it to more to congestion of games. Tammy's was more of an impact injury. That can happen at any time.
"We are congested, we are. It's the same for all teams, that's true, but when individuals are coming into play and people are getting injured, maybe that's something that should be looked at."