The £17million summer signing scored his first goal from open play in today's 6-0 FA Cup quarter-final win over Millwall.
"Yes of course [this could be turning point for Vincent]," he said.
"In the last few months he's stepped up and started to work better and I was happy in the way he was performing on the training ground.
"In football you need to be ready to show your quality. Janssen was working very hard, today was a great opportunity to score and he scored."