Benteke, 22, made an astonishing U-turn by withdrawing his transfer request and signing a new four-year deal on Friday.
But Villa boss Lambert said: "I knew what was going on from day one.
"You're asking someone who has a massive responsibility on his shoulders where he wants to play?
"All the speculation - Is he or isn't he? It's never nice. I had a good chat with him and he'll be a massive player for us.
"It's great because it puts it all to bed. He's only 22, he will make mistakes like everybody else. You heard the crowd today. They can't wait to see him back again, which is great."