The former England boss watched his new side held at home to Barnsley yesterday on the opening day of the Championship season.
"The 90 minutes just confirmed to us what we already knew - that we need to strengthen the squad before the end of the month," McClaren told the club's official website.
"The chairman is fully behind us. The backing is there but it is just about the process of getting players in. The good thing is that we still have time to do that."
McClaren, 50, became the Tricky Trees' manager in June.