"Soon I will look into the eyes of my players to make sure they have retained the hunger they need to succeed. Then we will look forward.
"If I don't see the appetite and ambition, then the bus moves on without anyone who has lost the spark."
Speaking in United's official yearbook, Ferguson added: "We cannot afford to slacken.
"The Premier League has never been so competitive and it is difficult to defend a European championship.
"But we have a group of players good enough to do it and we have the right back-up in terms of a strong squad."