The centre-half wants a first-team place next season to ensure he makes Australia's World Cup squad, but he struggled to hold down a regular spot last term.
Lambert told the Colchester Gazette: "I don't want to stop anyone from going to play in a World Cup. To go to a World Cup is the pinnacle of any player's career.
"But he has to play and we can't guarantee that - just like we can't guarantee that to anyone.
"This season will be an unbelievable one for him, because it's a World Cup year. If he doesn't play here, I'm pretty sure the manager there (Pim Verbeek) will say, 'hang on a minute, what's going on there?'.
"He has done really, really fine for me whenever he has played and he's been playing well at the minute for them."