The Red Devils were unconvincing in a 3-2 comeback victory over Cypriot minnows Omonia on Thursday night.
It followed a 6-3 hammering at Manchester City on Sunday, which illustrated the gap between United and their rivals.
Manager Ten Hag believes City are the gold standard when it comes to a constant winner’s mentality, while United still fail to bring the right attitude on occasion.
“It’s physical, it’s mental but it’s also sustainability especially,” said the Dutchman.
“When you had a lack of it in the last years, it’s not something that you build or progress in a week or a month.
“No, it has to be in your system. It has to be, but it’s a demand nowadays in top football.
“I think it’s quite clear that City is a standard in that and there are more teams, but I think also we can deliver that.
We have seen it against Liverpool, we have seen it against Arsenal, but now we have to do it on a consistent basis and that is what we have to work for now.
“But that will not come overnight. It will also take more than weeks, it will take months.”
Ten Hag urged his players to get ‘nastier’ – but with a degree of cunning – as the boss highlighted an early booking for Diogo Dalot in the Manchester derby to explain his point.
“I know nasty,” Ten Hag said. “I know what nasty is. I think also in that matter we can progress. Sometimes also play a little bit more smart.
“We have too many bookings and some in the start. I heard, yeah, in the Premier League they will play tough, but I’m really wondering why we collect so many bookings and I don’t understand.
“Like the first booking on Sunday, I really don’t understand that when we want to play tough, we want to keep the game going.
“Then in the second minute already it’s a booking for [Dalot]. I think it’s a duel, clear it was a foul, but then get booked? Also I saw many other bookings where I think ‘OK, is that necessary?’
“But also I take it to the players. I have to keep the mirror [on them] that they have to play smart, but also, hey, they have to play nasty as well.”