Millen said: "It's a bit unfortunate for Stefan. He came into us for Plymouth not as fit as we expected and it was a difficult start for him."
He also confirmed that midfielder Gavin Williams will remain on loan to Yeovil for the rest of the season.
Millen said: "He [Williams] isn't forgotten, but we have a lot of competition for midfield, so he might as well play more games for Yeovil."
The Welshman has made two appearances and scored one goal since joining Yeovil on 11 March.
Source: BBC Sport
Source: BBC Sport