The Austrian striker came off the bench to grab a brilliant brace inside six minutes at Norwich to leave the West Midlands side within sight of a Wembley final date in February.
The 21-year-old and fellow forward Christian Benteke have been keeping Darren Bent out of the side in recent weeks, but the England international started in midweek, only to limp off with a hamstring injury.
And Weimann believes that the competition for places under Lambert can only be a good thing for the midlands club.
He said: "You want the manager to have a headache because that is when you do well as a team.
"Everyone tries their best in training and whoever the gaffer picks we know will give their all. We are a tight group and nobody wants to let anyone else down."
Weimann conceded Bent would be frustrated after coming off with a hamstring injury and said: "He started and wanted to show everyone that he can score goals for us and still wants to play for us.
"But obviously it is disappointing that he has picked up an injury. Hopefully he can come back fit soon. It was unfortunate for Darren that he got injured, but for me it was good that I got on and was able to score two goals.
"He congratulated me on the goals, although he couldn't get up because his leg is injured."
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