Mannus had been sent off during Saints’ 3-3 draw with Hearts at McDiarmid Park on January 18 following an angry clash with Jambos striker Ryan Stevenson.
The keeper missed the league clash with Partick last Wednesday and was was set to complete his two-match ban against United at Tannadice last night. However, an inspection on the pitch carried out yesterday afternoon found the surface to be waterlogged and the call-off means that Mannus misses out this weekend.
Mannus admitted: “It is a massive game for the club but getting through to the final is all that matters.
Tam Scobbie is already banned for the semi-final against the Dons after being given a two-game suspension for a head-butt during the Hearts match.
St Johnstone signed midfielder James Dunne on loan from Stevenage yesterday and he, along with fellow new signing Chris Iwelumo, will make their debuts at Tynecastle.