United boss David Moyes was in Germany last week to watch Kroos in
action for Bayern and is understood to have met one of the 24-year-old's
representatives.
Kroos is out of contract at the end of next season and has refused to rule out a move away from the Allianz Arena.
And Guardiola has also suggested that the 24-year-old's long-term future may not be with the Bundesliga giants.
Speaking to reporters on Tuesday, the Spaniard said of Kroos: "He is an important player."
However, he added: "But it can go fast in football. Here today, there tomorrow."
United
have also been linked with Bayern defender Dante, who is also out of
contract in June 2015 and has recently confirmed he is aware of interest
from Old Trafford.