Carlos Carvalhal says there were positives and negatives from the 4-2 defeat at Scunthorpe on Wednesday night.
The Owls twice took the lead at Glanford Park only to see the hosts hit back to win the friendly fixture.
And Carvalhal told swfc.co.uk: “We mixed the good with the bad tonight and we have to rectify the mistakes.
“We conceded four goals and that is never good and we are upset about that. But we scored two goals and we could have scored four ourselves.
“As I have said before, the results are not important and I would say the same thing if we had won the game 4-2.
“For the players, pre-season training and pre-season friendlies are about increasing the fitness levels towards the game that counts, and that is when we play Bristol City at Hillsborough on 8 August.”
Carvalhal added: “We have one more friendly game to go before then and much training and I want to see the players react in a positive way in correcting the mistakes.
“But I do not want to talk too much about one game, I look at pre-season as a whole and the overall preparation for the new season in front.”