Reja was appointed last week following the dismissal of Vladimir Petkovic and saw Miroslav Klose grab the only goal of the game last night.
"We haven't played very well today," he told Sky Sport Italia. "We can do much, much more.
"These three points give us some serenity, but we know we are not in the best condition.
"Inter are a very good team and can take advantage of any errors you make, especially if you give them space out wide and in midfield.
"I thought we played a smart game today though."