A few years ago an Italian boy joined my grade 6 ESL1 class in August. He had a little German but minimal English. At first he was unhappy in school and refused to say anything. He soon became more comfortable and listened attentively in all his classes. Nevertheless he continued to be silent until well after Christmas. He then decided that he was ready to speak. Immediately he showed a good command of basic grammar, and in a very short time he became virtually fluent. Clearly, in his case, comprehensible oral and written input was sufficient to give him a very sound foundation in the language.
Of course, it could be claimed that he would have learned even more quickly had he been prepared to speak from the outset - and certainly, speaking to people usually means that they will speak back to you, thus increasing the amount of comprehensible input! But this does not refute Krashen's argument that comprehensible input is a sufficient condition for language learning.
Ensuring that their classes are comprehensible to ESL students is the single most important thing that mainstream teachers can do to aid those students in learning both the content of the class and the language in which the content is conveyed.
Further reading: What Teachers Need to Know About Language