After an impressive season for Harlequins, Brown started England's first Test of the summer tour to South Africa at full-back with Foden on the wing.
But for the second Foden returned to full-back after Brown was ruled out of the series with a thumb injury while in the third, Saracens' Alex Goode stepped in, allowing the Saints man to return to the wing.
Foden was scheduled to see a specialist on Wednesday and if he fails to recover in time for the autumn internationals, Brown and Goode would be vying for the full-back slot.
But 27-year-old Brown's thoughts were with Foden rather than his place in the side.
"It's really unfortunate for Fodes, who was just getting into a bit of form as well," said Brown.
"I enjoyed playing alongside him in that first Test against the Springboks. I thought we linked up well at the back and I was really looking forward to maybe continuing that partnership.
"It's good to play with someone who knows what a 15 wants. He's so good in the air, so good on the counter attack and he reads the game so well. I wish him a speedy recovery."