David Moyes traveled to Germany last Friday to open talks over a potential transfer for the German international.
Reports suggest United are ready to offer £25million to land the highly rated 24-year-old.
Guardiola was asked about speculation linking the midfielder with a move to Old Trafford, and did little to play down talk of a transfer.
“He is an important player but things can move fast in football,” Guardiola said.
“Here today, gone tomorrow.”
Negotiations over a new contract for Kroos appear to have stalled, with the players agent Volker Struth saying the terms offered by Bayern Munich are unacceptable.
That has alerted Manchester United, with Bayern considering cashing on a player whose contract expires in 2015.
Kroos himself is also thought to be open to a move.
“I am unable to rule anything out,” Kroos said.
“I am under contract with Bayern Munich (until June 2015). Actually there are yet no further talks (regarding a new contract) scheduled. I will therefore fully focus on the second half of the season.
“There are worse situations in life. I play for a top club, and under a top coach. I am pleased, and sometime in the future a decision will be made.”