Forrester was tormentor-in-chief in the Bees side that inflicted only the first defeat in 10 on an MK Dons side.
He scored one and laid on another in the 3-2 win, which saw Brentford leapfrog the Dons into fourth place in League One.
But the winger, signed by Rosler from Ajax, is out of contract at the end of the season and is becoming a hot property.
Rosler said: "He thrives on the attention of the fans here and the fact that he has a manager and a team that let him express himself.
"But his career only really started 17 games ago and he's not the finished article. I'd hate him to make the wrong decision and that's why it's so important that he realises what he has here and that the grass isn't always greener elsewhere. He has a big decision to make and he needs to look at what he has here."
Rosler added: "I never had any doubt we would come back because we had belief in our game, belief in each other and belief in our quality.
"We created plenty of efforts on goal and were worth the points. We pressured them well and were in their faces and that is the way to stop them plying their game, but we will be keeping our feet on the ground."