The German was in sombre mood despite Clayton Donaldson's goal just before the break having maintained the Bees' unbeaten start to the season.
The game was just five minutes old when a committed challenge from Adam Chambers saw him skid into the on-loan Everton youngster on the edge of the rain-soaked box.
The youngster was stretchered off and taken to hospital after lengthy treatment for a suspected broken leg.
Rosler said: "Everyone was in shock. He is a very promising young lad, a great character and he loves his football.
"Hopefully he will come back soon, but it is a very nasty injury and we will give him all the support he needs."
Rosler hailed the win over form side Walsall as a good three points and was pleased to "win ugly" after being forced to reshuffle his side so early.
Clayton Donaldson's headed goal just before the break was all that separated the sides, the big striker glancing home Shaleum Logan's right wing cross after 41 minutes.
Visiting keeper Richard O'Connell kept the Saddlers in it to the very end, denying both Donaldson and the influential Adam Forshaw in the final 15 minutes.
The keeper made reflex saves to thwart close-range efforts to deny the pair after some slick approach play on the glassy surface sodden by a pre-match downpour.
Forshaw was also unlucky not to settle the home side's nerves earlier on when his rising 30-yard rising drive cannoned to safety off the crossbar with O'Donnell beaten.
Button also came to his side's rescue late on when he bundled a Romaine Sawyers' header around the post as Brentford looked to attack on the break.
Rosler said: "I was very proud of my players for the way they dug deep as the game wore on. Sometimes you have to win ugly.
"Walsall are a good footballing side and they played through us a few times, but we defended the lead well and won even though I thought at times they were the better side."