מתני' בעל קרי מהרהר בלבו ואינו מברך לא לפניה ולא לאחריה ועל המזון מברך לאחריו ואינו מברך לפניו רבי יהודה אומר מברך לפניהם ולאחריהם:Or, in English:
MISHNAH. A BA'AL KERI24 SAYS THE WORDS [OF THE SHEMA']25 MENTALLY26 WITHOUT SAYING A BLESSING EITHER BEFORE OR AFTER. AT MEALS HE SAYS THE GRACE AFTER, BUT NOT THE GRACE BEFORE. R. JUDAH SAYS: HE SAYS THE GRACE BOTH BEFORE AND AFTER.Meanwhile, the Yerushalmi brings down an interesting brayta in this regard:
מתניתא במקום שאין מים וכר' מאיר דתני בעל קרי שאין לו מים לטבול הרי זה קורא את שמע ואינו משמיע לאזנו ואינו מברך לא לפניה ולא לאחריה דברי ר' מאיר וחכמים אומרים קורא את שמע ומשמיע לאזניו ומברך לפניה ולאחריה.Thus, the Mishna is like Rabbi Meir, that he may read the Shema but not have it reach his ears; but the Sages argue and say it must reach his ears. If you recall from before, Bavli Berachot 15a, there was a dispute between Rabbi Meir and Rabbi Yehuda citing Rabbi Eleazar ben Azariah:
R. Judah said in the name of R. Eleazar b. Azariah: When one recites the Shema', he must let himself hear what he says,10 as it says, 'Hear, O Israel, the Lord our God, the Lord is one'. Said R. Meir to him: Behold it says, 'Which I command thee this day upon thy heart': on the intention of the heart depends the validity of the words.11This is then a consistent dispute, about whether Shema itself requires that it reach his ears.
This assumes that meharher belibo is the same as korei ve'eino mashmia le'ozno. One could readily argue that the former involves no motion of lips or voicing whatsoever, while the latter involves some amount of speaking, just not to the extent that it reaches his ears.
Within our gemara on 20b:
GEMARA. Said Rabina: This would show that saying mentally is equivalent to actual saying.27 For if you assume that it is not equivalent to actual saying, why should he say mentally?28 What then? [You say that] saying mentally is equivalent to actual saying. Then let him utter the words with his lips! — We do as we find it was done at Sinai.29 R. Hisda said: Saying mentally is not equivalent to actual saying. For if you assume that saying mentally is equivalent to actual saying, then let him utter the words with his lips! What then? [You say that] saying mentally is not equivalent to actual saying? Why then should he say mentally? — R. Eleazar replied: So that he should not have to sit saying nothing while everyone else is engaged saying the Shema'. Then let him read some other section? — R. Adda b. Ahaba said: [He must attend to that] with which the congregation is engaged.It seems that the gemara (going into 21a), at least, is taking this idea of hirhur == actual speech in the general case, rather than a specialized din in kriat Shema.
But maybe we can draw some equivalences between Ravina and Rabbi Meir, who both hold that there is some sort of fulfillment here; and Rav Chisda and Rabbi Yosef / Rabbi Eleazar ben Azariah, that it cannot possibly be fulfillment of Shema.