Fredi Kanoute replaced Jermain Defoe with us training 1-0 with 10 minutes to go at Old Trafford.
In the end United ran out 3-0 winners, capitalising on the space left as we pushed forward with goals in the 89th minute and injury time.
Acting boss David said: "It's always difficult and I know some of the fans weren't quite happy and thought that maybe a third striker should come on and open the game up considerably.
"Stephane Dalmat was already on and sometimes you have to play the system that suits the three strikers from the start or have a different shape, perhaps three centre-backs.
"But it is a massive gamble and it can destroy your balance because all three strikers tend to want to get on top of each other and therefore you lose width.
"There was a purpose in swapping a striker for a striker and some people can't see that.
"It's never easy to pick a team but I have to say to all our supporters - who have been magnificent - that when we pick a team, many things are taken into consideration.
"That includes the balance of the team, how the players have reacted in training, how players combine with other players and whether or not the opposition has a player we have to take into account.
"For instance, the thinking behind starting Dean Marney on Saturday was because he would work back hard in terms of retrieving the ball and give us more possession, which he did well.
"It was a close game and it was horrible for us to lose goals in the last couple of minutes and lose 3-0. They shaded the game but there was no disgrace in that.
"We will continue to pick the 16 players that we consider - having worked with the players and knowing the players well - the best 16 to represent this football club."
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David Pleat has explained the thinking behind switching a striker for a striker as we chased a draw at Manchester United on Saturday.