I don't play Online however a few general defensive tips may help none the less. Don't hold R1 all the time as it often tires your player out and Side Backs get tired very easily so bare that in mind. Holding R2 can help also, this acts as a free-roaming control allowing you to manuveore your player wherever you want. For example, if the opposition's striker is waiting for the ball to arrive at his feet, holding R2 and moving your player forward can help barge past the striker and intercept the ball if you're quick enough. If you're really struggling against a winger and need help, hold down Square if another player is close as this will call him over for back up and can overcrowd the wingers space forcing him into errors and such. More than anything it's about positioning, you should often glance around the pitch just to check if your defenders are in good positions to clear the ball and prevent a goal.