Alonso told chelseafc.com: "From my first day at Bolton you could see he was a great player so I'm very happy for him and what he's gone on to achieve, I'm pleased to have a chance to play alongside him again.
"It's not easy when you move to a new country because of the changes in culture and language but I've been here before so I don't think that will be a problem."
On his ambitions, he continued: "I've played in different positions over the last few years due to a change in managers and systems, but I've always played mainly as a left-back, even if I can also play in left midfield or in central defence.
"My main aim is to defend well and I would say that's my biggest attribute, but I also like to help the team in attack. I'm tall so I can deal with high balls and I think I'm a good crosser, but the most important thing is to try to put all of that together in order to help the team."