Dervite, who has been on loan at Southend, is being linked with a move away at the end of the season and Redknapp added: "Possibly it is a case where we have to sell before we can buy.
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Spurs can still finish seventh but Redknapp has identified Goodison Park as the place where his team could step closer to the Europa League.
Spurs are a point off the valuable seventh spot which would guarantee Europe, but their run in includes only one more home game - against Manchester City, in the middle of away games at Everton and Liverpool.
Indeed, one point separates four teams chasing that seventh place with West Ham on 48 points and Manchester City, Fulham and Spurs all on just one point less.
"We'll keep going," said Redknapp. "The teams around us all won again (at the weekend), West Ham had a great result at Stoke, they'll have a great chance now with two home games left.
"Fulham lost at Chelsea but are right in the mix, City as well, it's tight. We'll have to win away and we've two tough away games in Everton and Liverpool, plus City at home.
"A top-half finish is more than we could have hoped for earlier in the season. Look at our points tally now and it's been a terrific achievement by the players."