Norway international Forren played the full 90 minutes of his country's 1-0 defeat to Albania in World Cup qualifying on Friday despite having not played a first-team game since December, in the UEFA Europa League with former club Molde.
The centre-back joined the Saints from Molde in January but has only played two under-21s games for the Premier League club up to now.
Despite his lack of game time, Forren declared himself raring to go ahead of the Albania clash.
"I feel in very good shape and I know that I will be ready when my chance comes along," he told the Southern Daily Echo.
"I have trained very well for two months at a high level. I have played two under-21 matches, where I have shown that I am in good shape.
"It was good to get those two games to confirm that. You can't start doubting yourself because you are not playing regularly."