Former Manchester United defender Stam, 44, was appointed Royals boss in June following Brian McDermott's dismissal.
"If you look at the league, there are teams in there who will make it hard to be in the top 10," he told BBC Sport.
Stam takes charge of a side who finished a disappointing 17th last season and start their Championship campaign at home to Preston.
"You can't expect us to end up in the top six," he told BBC South Today.
Stam has been appointed on a two-year contract for his first job in management and has made a number of additions to the squad.
But the former Netherlands international appears focused on a long-term strategy above an immediate promotion back to the top flight.
"We're concentrating and focusing ourselves on improving every game and to make ourselves better as a team," he said.
"It's not going to happen that quickly, you need to have a bit of patience as well."
Reading manager Jaap Stam is not expecting a Championship top-six finish in his first season in charge.