Lambert brought in seven players during the summer but Villa are still stretched in certain departments after the departure of several experienced men. They have four centre-backs on the injury list in Richard Dunne, Ciaran Clark, Nathan Baker and Chris Herd.
When asked about his plans for January, Lambert said: "The squad needs to get bigger as well. That is important as well. That's normal. Everyone has to have competition for places. That is vital."
Expanding on his point, Lambert added: "It is like the centre-half situation. We lose Ciaran and Nathan and have to put Matt (Lowton) or Chris (Herd) back there.
"It shows you are short in certain areas and you have to improve the squad which we will try to do."
Lambert is convinced Villa are heading in the right direction despite winning only one of their opening nine Barclays Premier League matches.
He said: "This was never going to happen overnight. A lot of people just expected us to come in and see a real change in fortunes.
"There is certainly progress, you can see that, but you want to keep going until you think, 'We're there'."