"He needs games to be ready for the first team and if that means playing in the reserves then so be it," Millen told BBC Radio Bristol.
"But if something comes up that appealed to him and us then we would look at it."
The Wales international arrived at Ashton Gate in the summer of 2009 from Derby County and made 37 league appearances last season.
And Millen was quick to deny that Nyatanga had fallen out of favour.
"He would have been told by now if he need to find another club and that's not the case," he said.
"I see him as competition on the left-hand side of defence.
"He's one that's in our development groups, even though he's played a lot of games, and is still young and learning."
Source: BBC Sport
Source: BBC Sport