Zenden, 36, revealed: "He was in a hotel in Doha in Qatar for a trainer's clinic.
"We spoke briefly and he wanted my contact details.
"Rafa called me on Wednesday evening and said 'make sure you are ready and be in London tomorrow before one o'clock as we're starting at Chelsea' then he hung up.
"This is an unbelievable chance for me at the very highest level, with a top club.
"It makes me proud that someone like Rafael has offered me this."