LSG vs MI today's match will be great as if Mumbai Indians can win the match then they can be guaranteed to go to the knockout stage.
The word guarantee is irrelevant in the game of cricket because anything can happen in the game of cricket. You said Mumbai Indians win today will make the playoffs absolutely certain for them . you are guaranteeing them to be certain but you may have failed to match some equation here.
No prediction can be made with certainty in cricket, anything unexpected can happen on a 22 yard pitch in cricket. If Mumbai Indians win today's match against LSG, it should not be guaranteed that they will only secure the playoff spot. But if Mumbai Indians win today's match then they will move to the second highest position in the points table with 16 points. If LSG win then they will overtake Mumbai Indians to third position and Mumbai Indians will move to fourth position. If Mumbai Indians really lose then their playoff spot may not be guaranteed. On the other hand, Gujarat Titans is the only top team that has confirmed the playoffs in this year's IPL. However, it will not be possible to determine today which team will confirm the playoff stage as the second team.