Schumacher, 42, has scored just 32 points in 11 races this year, and currently lies 10th in the championship, but he has made it clear he has no plans to leave the sport before next year.
Haug admitted Schumacher's qualifying form this year has not been great, but said his race performances show how hungry he still is.
"I think you could admit that in qualifying there is something missing, but his race performance this year, and I'm not speaking against [team-mate] Nico [Rosberg], but over the last five races the race pace was comparable, sometimes Michael is a bit quicker," Haug told AUTOSPORT.
"If you look at the overtaking statistics, Michael is leading, nobody has overtaken more than him. This is quite something, you could say. It'd be difficult for [championship leader Sebastian] Vettel, starting at the front all the time.
"I think you saw some very calculated braking moves from him in Canada and he is relaxed, and sometimes too ambitious here and there. But he is very ambitious, not too ambitions, but he is ambitious and the fire is burning. Look in his eyes and you will see that."
The Mercedes boss also believes there is no real desire for Schumacher or Rosberg to leave the team despite the failure to produce a winning race, but conceded no one in the team expected to have such a tough season.
"If we had told you at our presentation 'well, wait and see, it's gonna end the year with fourth place', you would have said 'come on, this is sandbagging or whatever. This cannot be true'," he said.
"So of course, we expected more, but can you be depressed afterwards? Of course you can, but we're aware of telling ourselves fairy tales, so we're working even harder, we have to have a disciplined approach to it, we have to take the right decisions, we need to understand the car...
"So I do not make announcements or noises, but we are working very hard and this is just the medicine for us that makes us work more and even harder."