The 27-year-old scored the Bianconeri's second of the game in Glasgow after Alessandro Matri poked home the opener in the third minute.
Marchisio admitted that it is unlikely that Celtic will be able to overturn the deficit, but said that his teammates must not take victory for granted.
"It is a very important victory, especially as we can be a little more concentrated on Serie A," the Italian international told UEFA.com.
"We haven't passed through yet, but it is a great win in a difficult arena and we put in a fine performance.
"It's true it is tough to overturn a 3-0 away win, but we must never be certain and always go out there with the same determination. It was crucial to come here and score a goal, but we ended up getting three."
The second leg takes place in Turin in two weeks' time.