Vieira sent the striker on late in the game and admitted afterwards the striker was upset being held back so late.
He said after his first game in charge on Sunday: "We still have to work on the offensive phase and these players have the quality to come in, move centrally and do well.
"Mario was disappointed at the end of the match because he wanted to enter the field earlier. But these are exactly the reactions I want to see in my players. The fact that he wanted to enter the field earlier shows that he wants to do well."
Vieira also said: "The atmosphere was exciting. I liked the atmosphere in the stadium. From a playing point of view, we played a good match, we have some things to improve.
"I felt sorry for the boys because we deserved to win this match because we were the stronger team in the second half. That's football, it's difficult to accept the second penalty."
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