Foster appeared to have been replaced as Saints’ flavour of the month after some horrendous errors earlier in the season, especially away to Bradford.
But Rush has revealed that Foster has to work hard in the reserve team to replace some of the in-form three quarters being selected for Super League.
Rush said: “We’re doing well for wingers at the moment so Jamie has to work hard. He has to do something special in the under 20s to knock somebody over in the first team.
He said: “Hull KR are playing well and alongside Wigan and us, are one of the form teams in the competition. We will need to make sure we are at our very best.
“The eight will be tight right up until the end of the season and it is up to us to keep winning, stay in there and not get dragged into a battle for fifth to eighth.”