Wanyama swapped Celtic for Southampton last summer, while Forster left the Hoops for the Saints ahead of the current season.
Neither have been forgotten by Mulgrew, however, who rates the pair as the best Celtic players he has played alongside.
"I've played in a few good Celtic teams but the one that stands out is the one that beat Barcelona," the 28-year-old wrote during a Q + A with fans on Sportlobster.
"Best player I've played with is hard, I would say Victor Wanyama and Fraser Forster are top players who played in that Barcelona game and did great for us".
And when asked specifically about whether Southampton goalkeeper Forster can become England's no.1, Mulgrew replied:
"There's no reason why not, he's a top keeper, everybody saw that last night, it's something we all knew here for a while.