The tournament hosts have already lost their first two Super Eights matches against champions Australia and Stephen Fleming's New Zealand.
A third loss may not necessarily eliminate them from the competition as they did bring two points through from the group stage.
But defeat would make their task considerably more difficult and would certainly demand victories from all their remaining matches against Bangladesh, South Africa and England.
"It's going to be a huge game because Sri Lanka is playing very good cricket, they had a hiccup against South Africa but they showed fight right to the end," Lara told reporters at the Providence Stadium.
"The guys understand the position we are in at present having lost against New Zealand and Australia. This match is of utmost importance and we have to put our hands up and be counted.
"We've never had the attitude that because we have six games we can afford to lose one or two. We try to win every game. It's important to have the mindset where we go out there and play each game like it's a final."
Lara said the whole 15-man squad was fit and team selection would be decided the evening before the match in a team meeting.
The team's biggest issue was finding runs from a talented line-up that was yet to fire throughout the World Cup.
Lara's number in the batting order has been the subject of debate and he said that would be discussed in a team management meeting, also.
Sri Lanka, meantime, won all three of their group matches including victory that sealed India's elimination.
However, they lost their first Super Eights game against South Africa despite Lasith Malinga's late burst of four wickets in four balls.
Therefore, they too would see this match as a must-win game and would not want to start their Super Eights campaign with two losses.
"I'm not sure what the morale is like in the West Indies camp but if it's down we'll try to take advantage of it," coach Tom Moody told reporters.
"I'm not sure of their position. They've come off two losses so it's a big game for them but it's equally as big for us.
"We're going in there all guns blazing hoping to play our A game and get another two points on the board."
West Indies: Brian Lara (capt), Ramnaresh Sarwan, Chris Gayle, Dwayne Smith, Shivnarine Chanderpaul, Dwayne Bravo, Marlon Samuels, Ian Bradshaw, Corey Collymore, Jerome Taylor, Denesh Ramdin (wk), Devon Smith, Lendl Simmons, Daren Powell and Kieron Pollard
Sri Lanka: Mahela Jayawardene (capt), Kumar Sangakkara (wk), Sanath Jayasuriya, Upul Tharanga, Marvan Atapattu, Tillakaratne Dilshan, Russel Arnold, Chamara Silva, Chaminda Vaas, Farveez Maharoof, Lasith Malinga, Dilhara Fernando, Nuwan Kulasekara, Muttiah Muralitharan and Malinga Bandara