Foster, making his first start of the season, proved the difference between the two sides as he ended the match with 18 points.
After a tight opening 20 minutes it was St Helens who struck first with Foster claiming his first of the evening on 21 minutes.
The hosts started the second half strongly and took the lead on 45 minutes through debutant Zak Hardaker. However the former Featherstone winger had a helping hand from the touch judge who inexplicably missed a blatant forward pass from Kallum Watkins.
Saints hit back with a try from Michael Shenton five minutes later and Foster’s conversion put the visitors 12-10 in front.
However Paul McShane, another youngster to impress on the night, barged his way over the line from close range on 56 minutes to restore the Rhinos lead.
Both sides were eager to keep the ball alive but it was St Helens who were to prove the more clinical.
St Helens defended superbly throughout and they secured the win on 77 minutes in spectacular fashion. Meli offloaded to Foster who spun the ball around his body before touching down in the corner for a fantastic try.
The visitors will be encouraged by the mature showings of Gaskell and Lomax. The performance of the latter suggesting that Saints coach Royce Simmons will not have to search too far for Eastmond’s replacement.