"As part of our preparations we have to assess the opposition and we saw their performance in the last game against Braga," Ferguson is quoted as saying on United's official website.
"[Before that] they knocked out Basel, beating them twice. We know from our own experience last season that Basel knocked us out. That gives us every respect for Cluj because it tells you they're a good team.
"Cluj defend quickly and get plenty of bodies behind the ball. The main strength is counter-attack, as they showed against Braga."
In the other Group H game on Tuesday, Galatasaray will host Braga.