The midfielder is currently enjoying superb form with the Hornets.
Deulofeu said: "It was a good time, because I was there for many years, but it was also complicated because I was very young and I had to compete with the best players in the world in a stadium like the Nou Camp.
"When you are so young, it's a bit difficult. You need a little confidence, and the only one who explained something to me is (Pep) Guardiola. It takes a lot of things to succeed there, you cannot live in the past. If you do, you're dead."