The 59-year-old, who has previously managed in Serie A with Fiorentina and AC Milan, said that both teams had chances to grab a second goal in what was an "even" game.
However, he admitted that his side allowed Die Knappen back into the game following a promising start to the contest.
"We went in front with an early goal because of our good play, but we unfortunately made a mistake just before half time that led to Schalke's equaliser," Terim told UEFA.com.
"We had previously had chances to score a second goal, although Schalke also had openings in the second half to go in front themselves. It was an even game."
The return leg takes place in Gelsenkirchen in two weeks.