Nigel Clough highlighted the impressive performance of teenager Harrison McGahey in the disappointment of losing at York City on Saturday.
In the final senior pre-season outing, the Blades slipped to a 2-0 defeat at Bootham Crescent.
"There weren't too many positives, apart from Harrison McGahey," said Clough.
"Forget about his honest mistake for York's second goal, he will learn from that. He's 18-years-old, he won headers, got tight and wanted to get out and press."
Clough added: "We had two good games this week, against Huddersfield and Fenerbahçe, but were disappointing at York, the games caught up with us. We had tired legs and tired minds, our decision making was unbelievably poor.
"It is difficult to put our finger on why we were so poor, generally. We have been carrying a few knocks - Flynn, Wallace, Scougall, Baxter - it is not ideal, we'd have preferred more of them to get closer to 90 minutes, but it wasn't possible. We will monitor them all this week to try and get as many out there as possible when we start against Bristol City.
"There wasn't too much to see in the final third, apart from Chris Basham glancing a header just wide in the early stages."
United's pre-season fixtures have yielded two wins, a draw and three defeats.