Falcao has suffered an injury-hit start to his loan spell at United, scoring just one goal in five appearances since his transfer deadline day move from Monaco.
He is yet to complete a full game for United and has been unavailable for the last month because of a calf injury he sustained in training.
It will take at least another fortnight for him to return to action and van Gaal won’t rush to take up the option to buy Falcao because of his latest injury, which comes 10 months after he damaged cruciate ligaments playing for Monaco.
“Do you think we expected he’d be injured for so many weeks? Because of that (injury setback) maybe we didn’t buy him,” van Gaal said.
Falcao is one of the club’s top earners, but his only meaningful contributions on the pitch so far have been an assist for Robin van Persie’s goal against Leicester and his own strike in the victory over Everton.
And his manager says he has had no option but to keep the 28-year-old out of his squad to avoid making his injury worse.
“I don’t give any chance to players when they are not fit, believe me. Look at my record and you will see that,” he said.
“You have to be fit. And what’s your name? It doesn’t matter — you have to be fit. I’m sorry.
“I have to manage with the other players and we believe in these players. And we shall show it.”
When asked whether he expected to be justified in his decision to sell Danny Welbeck to Arsenal and loan Falcao, Van Gaal said: “I hope so. That is clear, I think.”