The former Blues boss was back at Cobham this week.
"I noticed that for Frank it was very easy for him to arrive in the box and score a goal. I liked both him and Michael Carrick because they played together at West Ham but we didn't have the money to buy both," he told chelseafc.com.
"I chose Frank because in my opinion the players who arrive in the box without the ball and score a goal are very difficult to mark, very difficult to see when they arrive, and Frank has a big, big brain for football.
"When you say something to him, when you say look Frank, you have to do, this, this and this, he understands immediately and this is important for a manager."