The Tottenham manager sees City as his side's main rival for fourth place in the Barclays Premier League, but believes the loss of Toure could be critical.
Drugs shock: Kolo Toure
Redknapp said: 'If it's an important player, it can have an effect on the team, for sure.
'They'll certainly miss him, although he hasn't played so much recently. But it just shows you, you've got to be so careful. You shouldn't be taking drugs.'
Tottenham's 1-0 win at Eastlands in May last year saw them reach the Champions League qualifying stage at City's expense and Redknapp thinks there may be another shoot-out for fourth spot when City host Tottenham on April 17.
He said: 'I've got a feeling it'll be between us and Man City. It could be that tight again - it depends who has a good run from now until the end of the season.
'It's that thin dividing line. There's nothing between the teams this year, is there?'
Battle of the bosses: Spurs manager Harry Redknapp (left) and City counterpart Roberto Mancini (right)
Redknapp suggested Gareth Bale and Rafael van der Vaart could be fit for the second leg of Spurs' Champions League clash with AC Milan on Wednesday night. Bale might even feature against Wolves tomorrow.
Redknapp said: 'Bale feels fine but he just needs to push himself through it now. He came out on Friday for a session with the fitness coaches, he'll hopefully train with us and we'll see if he is available for Wolves.
'I don't think that Rafa will be fit but there is a chance that he will be back for Milan.'
Toure finds a friend in Wenger after Manchester City defender fails drugs testToure stays away from Manchester City training as defender speaks to legal team over failed drugs test
Explore more:People: Kolo Toure, Roberto Mancini, Gareth Bale, Harry Redknapp Places: Milan