Mata scored on debut in victory over Norwich City.
Lampard said: "It's exciting for us to see top players like Mata and Lukaku coming in. They bring a freshness and a different angle to the team and they both came on and showed their quality.
"I've seen Mata in the Spanish League and he's a fantastic player, great first touch and great awareness. We saw that in his little cameo and we saw what a powerhouse Lukaku is and he's been a real breath of fresh air in training."