Normally you must unlock the best teams to use them in manager mode. This usually costs 5,000 points in the fan shop. However, you do not have to do this. To be any team in career mode, for example, Chelsea, go to the team management screen and select Chelsea. Change the team so that all the worst players are playing in the starting line-up and all the best players are in the reserves. This will make the teams star rating go down. If it goes below 4.5 stars, you will be able to select that team to use in career mode.