The Germany international has 18 months left on his current deal, and speculation intensified recently when Manchester United manager David Moyes was seen at a Bayern game sat next to a representative of Kroos.
The 24-year-old has rejected a new deal offered by Bayern, and he appeared unhappy when substituted after an hour of the Bundesliga leaders' last outing.
Bayern trailed by a goal when Kroos was replaced but they fought back to beat Stuttgart 2-1.
Guardiola said: "He is an important player but things can move fast in football. Here today, gone tomorrow."
The midfielder has been capped 41 times by his country, and made his Bayern debut in 2010.
Sky Bet have cut the odds on the German making the move to Old Trafford in January to 5/6.