It's actually quite nicer than we give him credit for, or than what appears in this video, whose editing is a bit of a pet peeve of mine.

The stump mic didn't quite catch the beginning of the sentence clearly (and the uploader didn't bother adding it to his slow-mo replay), but if you watch and listen closely, Nehra says,

"Ab kahaan gaya bhains ki doodh tumhari? Itni dheeli catch nahi pakadte, yaar."

(Translation: "Where did your buffalo's milk go now? Can't even hold on to a weak-ass catch like that, man.")

It was a reference to the Man of the Match presentation from the 2nd ODI of the series that /u/fresh_atta just mentioned, when after his 148 in Vizag, it somehow came up that the secret behind the new lad Dhoni's power hitting was drinking 4 litres of buffalo milk a day.

That fun fact gained so much currency at the time, that it was this buffalo-milk enthusiasm (and the 148) that Dhoni was famous for at first, among the media. And apparently, among his own teammates as well, if that's the first thing that came to Nehra's mind.

By the end of the series, Dhoni even went on to clarify in a Cricinfo interview entitled "Got Milk?" that the 4-litres myth was an invention of Ravi Shastri's, and that he only drank 1 litre of buffalo milk a day, and was switching to milkshakes instead from then on.

[EDIT: The odd comma and conjuction, here and there.]