Louis van Gaal has said he is not interested in whether Radamel Falcao is frustrated at being used only as a substitute and that the striker is still not fit to start Manchester United’s match at Southampton on Monday.

Falcao, who is on loan from Monaco, suffered a calf injury in October that ruled him out for over a month. Since returning, the Colombian has been only among the substitutes – for last weekend’s win against Hull City and last Tuesday’s victory over Stoke City.

In both those matches Falcao was used as a second-half replacement and has failed to add to the one goal he has scored since joining on 1 September. Van Gaal, who said the 28-year-old will again not be in the starting XI at St Mary’s, said: “I’m not interested. He has to follow my philosophy. Do you understand that? Yeah, I’m here because of that.”

Asked if it was down to a lack of fitness, Van Gaal said: “Yes, match fitness. He has trained now maybe two days after the match [with Hull]. So we had the match against Stoke on Tuesday when he played for 15 minutes. Then we had the training sessions after Tuesday. And before Tuesday I didn’t train – I trained the box exercise. You know the box exercise? Yeah? It is not a training session.

“We had to play again within two days after the Hull City game, then I have to recover the players. Then I cannot train him. After Tuesday I can train him. And we have trained very well on Wednesday. Then Thursday they are off. I can say to him to train but, no, we have trained before that.

“Then on Friday we had a training session. So he has trained in two training sessions. But a bigger contribution is to play in the second team for 45 or 60 minutes or even 90 minutes – what Jonny Evans has done already. But I cannot do that because I need him because [Wayne] Rooney was injured [for Stoke]. James Wilson was injured also [for Hull]. I have to adapt to the needs of the selection. Then I selected him in spite of the fact he can only play 20 minutes at that level because the rhythm of the game in the Premier League is the highest.”

Van Gaal reiterated the importance of players proving their fitness in training when he said Phil Jones and Rafael Da Silva were close to being available following respective calf and groin problems. “Yes, we are seeing the light at the end of the tunnel. Jones will train with the team next week if everything is good. Rafael has already trained with us for one week. Also now he has to play in the second squad. I’m sorry for that but it is my philosophy and the players [should understand]. If they don’t I’m not interested.”

Van Gaal again refused to expand on his relationship with Ronald Koeman, the Southampton manager, after the pair fell out when at Ajax in 2004. “You are reading that in the papers. But, no, I don’t want to talk about it. Why are you asking the question for the fourth time when I have given a clear answer. Why? Why? It is only your interest to ask,” he said.

Van Gaal did concede unhappiness at United failing to kill off teams. “I cannot deny that. This team is capable of doing that. We have to dominate more. Against Hull we dominated and that is why I was so pleased. We could have scored more goals. But 3-0 is a very good result against a very defensive team. I would always like to play like that but that is not possible.”

United are in fourth position, 11 points behind Chelsea and one behind Southampton, though the manager was reluctant to offer a rating of his team. “After one year there are a lot of aspects that I want to improve. Because I am a perfectionist, it is always what we can improve. It is never 100% all right. Never.

“Against Hull City I thought it was a rather good performance. I was very pleased. But against Stoke it was not so good. So within three days there was a big difference. And we are looking for consistency. I said already that we have to wait and see about the momentum – the media are asking about momentum – I don’t think so, because when you have such a difference between two matches in three days you are not in a momentum.”