Cech endorsed Chelsea's pursuit of "great player" Rooney and knows nothing can be discounted in football transfer business.
"Wayne Rooney is a great player but at the moment he's a Man United player," Cech said.
"We'll see what's going to happen. He's a great player. Let's see.
"You always want to have the biggest and strongest squad so you can challenge for titles.
"That's why you have a manager and sporting director to take care of that and you have to do the most important part and that's the work on the pitch.
"We already have a few more players and a great squad. If the manager doesn't think we have enough he will try to get players in. It is only his decision.
"For players, we concentrate on getting fit and doing our best, to play as much as we can. That's all we can do.
"In football things are changing every day with every squad. As time goes on, every team can make some changes. Anything can happen."