The former Tottenham boss told Abendblatt: "Nigel is key to us, no doubt. And the offer from Manchester City tempts him, although I think the most attractive thing for him is playing Premier League football.
"If he leaves us, I feel that we might not just buy a player for one position, I think we will be signing two or three players. It's clearly what we want to do. Losing Nigel is too big a loss."