The tactician fielded a makeshift starting eleven in order to rest some of his key players ahead of the visit of Udinese this weekend in the Serie A.
However, the Azzurri were convincingly beaten by the Dutch giants leading Mazzarri to lament his squad's lack of strength in depth.“We started well, but then lost the ball too many times in midfield. It is a game we have to put behind us,” he told reporters.
“We have realised squad rotation is necessary for the position we are in. It is the only way we can address all our commitments.
"If we played our best team, we would have paid for it on Sunday against Udinese. The fact is, there aren’t enough players to cover every competition.
“Either we drop points in Serie A or we drop them in Europa. Now we have to think ahead to Udinese this weekend.”
The former Sampdoria boss gave a chance to a couple of his younger players but admitted they still need time to develop.
He added: "The younger players need to mature. They need to grow. We want to focus on young players. People complain that Italy does not give a chance to youth players. However, this is what can happen when you do give them a chance, you end up losing points."