The 36-year-old says the Brazil international still has a lot to prove and believes Jose Mourinho has been a disappointment to Real Madrid supporters
All eyes have been firmly focused on the 21-year-old in recent days after sealing his long-awaited switch to Camp Nou, with the youngster set to be unveiled to the public on Monday.
Following the news of Neymar's imminent arrival, a series of players and personalities within the game have predicted future success for the star, but Guti is refusing to board the bandwagon until the youngster justifies his reputation.
"He is a great signing, but you have to wait and see how he adapts to the European game," he told Sport ES. "Both Madrid and Barcelona have had those big signings in the past and they have not proved as effective in Europe as other continents.
"He has shown what he can do in Brazil, but now he has to prove he can do it here because European football is very different."
The 36-year-old, who spent 15 years with the Blancos, went on to address Jose Mourinho's imminent departure from the club and believes the Portuguese "has been a disappointment to all Madristas."
"We expected a lot from him. Not only titles, but a little more humility would have been good for everyone and the club. He has been a disappointment to all Madristas.
"I wish him the best in the future. I have always said he's a great coach and I'm sure he would have won more with Real Madrid had he stayed a little longer."
Guti went on to approve of Real Madrid's reported pursuit of Tottenham Hotspur star Gareth Bale.
He added: "I think Real Madrid is a smart club who always want to sign the best players, and he [Bale] is one of them."