Petr Cech has become the first Chelsea player to fling his support publicly behind the club's bid for Wayne Rooney as the Londoners prepare to submit an improved offer to Manchester United for the England striker.

Their opening bid, worth around £20m, has been knocked back by the Premier League champions, who have indicated an unwillingness to sell the 27-year-old to a direct rival for the title. Rooney is understood to favour a move to Stamford Bridge, having grown disaffected at Old Trafford after falling down the pecking order behind Robin van Persie, but will have to hand in a formal transfer request to force his exit.

An improved offer closer to £25m is anticipated from Chelsea, with Arsenal also to step up their own interest in the forward, though it may not emerge until United's stance is clarified once Rooney has submitted that request, waiving certain bonuses due over the final two years of his contract in the process.

Regardless, it now seems unfeasible that the forward, who returned early to Manchester from the champions' pre-season tour last week for treatment on a hamstring injury, could remain at the club given he has effectively burned his bridges with the United hierarchy.

He would find the welcome warmer at Chelsea, where he would join André Schürrle, Marco van Ginkel and Mark Schwarzer as a new arrival, with Cech reflecting the general sense of enthusiasm – if also a certain caution – following confirmation of the Europa League winners' interest. "Wayne Rooney is a great player but, at the moment, he's a Manchester United player," he 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 all you [as a player] have to do is the work on the pitch. We already have a few more players in 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. In football things are changing every day with every squad. As time goes on, every team can make some changes. Anything can happen."Jose Mourinho was not offering any comment on the club's pursuit of Rooney at Chelsea's welcome press conference in Kuala Lumpur on Thursday evening ahead of Sunday's game against a Malaysia XI. However, when asked by a local journalist how a manager motivates players in a World Cup year, he did offer opinion on players, like the England striker, who find themselves potentially as a second-choice at club level.

"I always say to my players that when they are with the national team they don't think about our club," he said. "When they are with the national team they have to give everything for their team and country and forget their club. But when they are with the club they have to forget the national team and give everything to the club. That's the way I think.

"At the same time, normally players who want to go to their national team and play for their country, they must be first choice players for their clubs and be playing regularly. That's the big motivation if you are thinking about playing at the World Cup. If they want to go to the World Cup and they're second-choice at their club, then they're in trouble, they're in trouble. I think it's a crucial year for them. Many national team coaches are worried because they want players to play regularly for their clubs. You must play for me to play for you, you must play for you to play for me. It's a combination. They must share the situation."