The Red Devils took a massive step towards winning the title with a 1-0 victory over Sunderland, which keeps the gap over rivals Manchester City to 15 points with just eight games left to play this season.
Buttner told MUTV: "If we want to be champions, then we have to win every game. We have to focus every week and win every game until the end. If we are champions, then we can relax.
"We are in a good position but we have to try every week to play hard, not have any easy thinking and work hard for each other."
United's attentions turn to the FA Cup later today when they face Chelsea in their quarter-final replay at Stamford Bridge.