The 22-year-old let in a long-range effort from Dean Bowditch during Monday's 2-0 defeat.
The club's manager told BBC London 94.9: "Simon has been terrific for us the whole season. He is a young player who is allowed to make mistakes, although hopefully not too many.
"I will not blame him at all as he has been fantastic. We are winning together and losing together. I will be responsible for the result. I was disappointed with one or two players but the majority did very well.
"I will not analyse each player in public but there is a reason I make substitutions. Nobody will be blamed in public, although obviously in the dressing room I said what I had to say."
The result has left the Bees third in the League One table, outside the automatic promotion places.