The 29-year-old came under fire for his performance against Newcastle United, in which he conceded twice in a 2-2 draw at St Mary's.
Pellegrino, though, does not think that one bad game should affect Forster's standing as a good goalkeeper.
"The goalkeeper needs confidence," the Southampton boss told The Express. "Three or four games ago everybody was saying he has to come back to the national team and is one of the best goalkeepers in England. I have the same opinion. Fraser is a really good goalkeeper.
"The position of the goalkeeper is always exposed in this type of situation but nobody remembers the good situations. Always when you make a couple of mistakes everybody will talk about this. This is part of our job. It is part of Fraser's job. The important thing is how you react to this situation, how you achieve this moment."
The coastal club currently reside 12th in the Premier League table.