Speaking to the Formula1.com, Whitmarsh believes that Hamilton will look back on his decision as the wrong one.
"I was surprised [Hamilton left], but I was not shocked. He told me right after Singapore and I am pretty sure he hadn’t made up his mind until after Singapore - the Monday or Tuesday after Singapore. I think it is always bad to make a decision in the aftermath of a bad race", he said.
"He was pretty sure that he was going to win that race and it was a disappointment and as I just said it is never good to make a decision in such a situation. I respect his decision, but I believe that he would be better off with us - we are the stronger team - and we intend to beat him next year!"
Whitmarsh was asked about Hamilton's comments about moving to a 'big team' and how that reflected on McLaren.
"Ah, you have to justify your decision. He is not going to say ‘hey, they offered me more money’. He is also not going to say that he’s made an awful mistake. I hope he thinks today that he’s made an awful mistake and I hope he thinks that next year. He’s made that decision and he has to live with that decision".
"I have known him since he was 11 and worked with him since his teens and I know we will both be very emotional after Brazil. We have had one or two emotional moments since the decision was taken and I believe, but you must ask him, that we have a very good relationship".
Whitmarsh was pressed further on Hamilton regretting his decision to move from McLaren to Mercedes.
"I think he has on occasions, yes. Probably, when you make a decision you have to tell yourself that the decision is made so you have to look forward. You say okay, that it is in the past so you don’t spend too much time thinking about why. You just look forward and make the best out of the new situation. Right now we still want to win races. We are motivated by that and our conversation circles around that. And maybe he is completely dispassionate about it, but my guess is that we both will have very emotional moments in Brazil. When the separation has arrived".
Whitmarsh also added that he is still unsure as to why Hamilton left the team and believes it is not financially motivated.
"The simple answer is that I don’t know. Lewis would be able to answer that question much better than me. If I were speculating there are a number of factors and the prime one, I think, is that there comes a time when a man feels he has to flee the nest. I think it was a bit of that and a range of different emotions. He has been with us for so many years that I do not fully understand it."
"We made Lewis an offer - an offer, which I believe is more money than any other driver at the moment is being paid. That leads us to suspect that our competitor and our partner Mercedes-Benz offered a bit more money. I don’t know that, but I think for Lewis made his decision. I am disappointed in one sense, but you have to focus on going forward".