The clubs had been due to meet in a televised Premier League fixture on Sunday, March 31, but have seen the game rearranged because of the FA Cup fixture mess.
Chelsea and Manchester United face a quarter-final replay in that competition, after last weekend's 2-2 draw at Old Trafford, but had no obvious dates in which to play it, due to international fixtures and the Blues' Europa League involvement.
To resolve the pile-up, it has been agreed that the cup tie will be held on Monday, April 1, meaning the encounter between Saints and Chelsea has been bumped from the Sunday back to the Saturday, with a 3pm kick-off.
Saints have made it clear they are unhappy with the decision.
Source: Saints Mad