Tandy's men must topple the Tigers to have any chance of keeping alive their quarter-final hopes. Leicester are currently a point clear of Toulouse, who entertain Treviso, with the Ospreys a further four points back in third.
An Ospreys victory could see them retain a chance of topping the group, but there is no margin for error at the Liberty Stadium this weekend, and Tandy said: "This is as big as it comes. It's a massive game and it is one that we can't afford any slip-ups in."
Tandy, who has seen centre Ashley Beck join an injury list that already featured his fellow Wales internationals Alun-Wyn Jones, Ian Evans and Aaron Jarvis, added: "We've got to be absolutely spot-on in everything we do - our preparation, our attitude and our attention to detail - if we are going to get anything from the match.
"We've done well over the last few weeks to put ourselves in contention in the PRO12 and in Europe, but we realise that we will need to go up another level this weekend in terms of performance if we want to get the result we need to stay in the competition.
"It's a tough group with six Heineken Cup titles between Leicester and Toulouse, and we are up against teams whose budgets blow us out of the water. But we have got a competitive group of players who will give everything for the cause, who never know when they are beaten, and we are looking forward to this contest.
"They (Leicester) will come down here confident after some good results of their own lately, and we know that we are going to have to match them physically.
"They've got the advantage in the pool, as it stands, but there is everything to play for."
England quartet Manu Tuilagi, Tom Youngs, Dan Cole and Geoff Parling all return for Leicester, while fly-half Toby Flood also starts after he was cleared at a Rugby Football Union disciplinary hearing earlier this week following his citing for a dangerous tackle in last Friday's Aviva Premiership win at Worcester.
Elsewhere, Munster will maintain their hopes of progressing from Pool One if they beat Murrayfield hosts Edinburgh.
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