The 26-year-old, who helped the Dutch side win the league and cup double last season, will join the Premier League team in the close season after the two clubs agreed an initial deal for €40 million on Thursday.
"He's certainly a player we hope can bring that bit of creativity and something different for us," Lampard told a news conference ahead of Monday's league game against Manchester United.

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"He scores goals, assists... he's got a great left foot and plays generally off the right side but can play behind the front man. Probably when you look at us this season, there have been game where we may have maybe struggled to unlock the door.
"I know he's not coming now but it's exciting for us as a club and the fans to know that he'll be here next season... I think he's a fantastic player."
Chelsea, who successfully appealed a one-year transfer ban, did not sign any recruits in January and Lampard was asked if more arrivals were expected at Stamford Bridge.
Veteran wingers Pedro and Willian are out of contract at the end of the season.

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"I don't know. We have to worry first about where we're at," added Lampard, whose side are fourth in the standings on 41 points.
"Trying to come in the top four is the priority for us so my main concern is that. But we're always on the lookout and we're always working together to see how we can improve.
"Those improvements could be needed maybe in the summer and we'll try to make sure we get the right ones."
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