Mata scored on debut in victory over Norwich City.
Lampard said: "Itâ€™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.
"Iâ€™ve seen Mata in the Spanish League and heâ€™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 heâ€™s been a real breath of fresh air in training."