The Dutchman has joined the Blues on a short-term deal until the end of the season and will combine his role with his current job as Russia boss.
“What happens after May is rather clear, because it’s rather difficult to do the two jobs,” said Hiddink.
“I have my obligations also in Russia,” he added, noting that the demands of the Premier League mean that it would be impossible to combine the two jobs.
“It’s rather difficult to do that because the Premier League is really challenging.
“We can do it now because we have just one or two games until June with the national team of Russia, that’s why we said OK. We can do it now and we can do it full-hearted and fully committed.”