I wasn't sure I'd need it.中I'd need it.是虚拟语气吗？

回答如下：

I'd=I would 因此完整句子如下所示

I wasn't sure (that) I would need it 

参考译文：我不确定我会需要它

以上不是虚拟语气，属于形容词宾语从句，主句为过去时，从句也要用过去将来时态。

(1) I am not sure that I will need it.（主句为一般现在时态，宾语从句一般将来时态）

(2) I was not sure that I would need it.（主句过去式，从句用过去将来时）

以上句子属于第2种情况。