Louis van Gaal criticised Adnan Januzaj for losing possession cheaply even as the Belgian sent Manchester United top of the Premier League with a winner at Aston Villa.
Januzaj was a surprise inclusion in the United team and Van Gaal praised the 20-year-old for scoring a "wonderful goal" in United's 1-0 victory but found fault with other aspects of his game.
And the Dutchman, who was similarly critical of Memphis Depay for giving the ball away, warned his scorer that the game consists of much more than just putting the ball in the back of the net.
"He had also a lot of ball losses and in my opinion [they were] unnecessary," Van Gaal told Sky Sports.
"Memphis Depay, a different player, has a lot of unnecessary ball losses. That is why we don't create many chances and we have to improve."
Van Gaal said in his news conference on Thursday that Januzaj is not for sale and the young forward stated he wanted to stay and fight for his place.
"Yes, but it he also has to show that," Van Gaal said. "He scored a wonderful goal but in all the other aspects of football you have to show it. I am very pleased of course because we have won on this goal but there is more than making a goal."
United have only had three shots on target in their first two games this season but Van Gaal was not concerned.
"That does not bother me because you have to score one more than your opponent and that is the result," he said. "We don't give many chances away and we have every game more chances than the opponent."
United have begun the season with back-to-back clean sheets and Van Gaal was satisfied by how few chances Villa had.
Van Gaal was pleased with the way his defence dealt with target man Rudy Gestede, who scored Villa's winner at Bournemouth last week and again came off the bench.
He said: "Aston Villa brought this man Gestede in and then the long balls but I think Smalling won every header and they only had one chance which is very good, I think."