Foster is in line for a return to the United team for the Champions League clash with CSKA Moscow at Old Trafford on Tuesday night so that Sir Alex Ferguson can rest Van der Sar ahead of next week's clash at Chelsea.
Ferguson said: "Ben was always going to lose his place as soon as Edwin was fit again. I could never dismiss his quality and experience.
"Edwin isn't showing any signs of decline. He's at an absolute peak. It's phenomenal.
"The future belongs to Ben, of course, but Van der Sar is still top dog."