The Colombia international midfielder has been heavily linked with a move to Chelsea for some time.
They are not the only side to have expressed an interest in the 27-year-old's services, but are believed to be the ones leading the chase.
No announcement of an official approach from Stamford Bridge has been made, but Thohir says Guarin's head has been turned and concedes that he may soon be heading for the exits.
He told Indonesian site Liputan6.com: "Guarin is an excellent player and we'd like to keep him, but when there are offers from big clubs you need to take it into consideration.
"We can't force him to stay if his heart isn't in the club."
Thohir is eager to rebuild Inter and their fortunes, but he has conceded that they will need to sell in January before recruiting - suggesting that they may look to cash-in on Guarin.
He told Republika Online: "The January window comprises of a series of opportunities in the market.
"Fans should not expect bombs but, if we have our chances, we will do something interesting.
"In any case, we have to sell before making any purchase.
"Our project is clear, there is no doubt about this. We want to build a healthy club, competitive in the industry, so this year we are going through a state of transition.
"This is not year zero, this is the last year of a fantastic era. Starting next season we will begin a new era."