Lambert has won just one match since taking over at Villa Park, and the Scot believes that he needs more options in order to increase competition for places.
He highlighted defence as a particularly weak area, having been left with just one recognised central defender for his side's 3-2 win over Swindon Town this week.
"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," said Lambert.
"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.
"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'."