The former Arsenal winger has just recently returned from a muscle injury and played a part in Villarreal’s 1-0 win over Deportivo La Coruna at the weekend.
“The game on Wednesday against United is a different story altogether to the Depor match,” Pirès told fifa.com.
“We are facing one of the best teams in Europe and it will be a very intense match.
“It can be difficult to maintain such a high level of concentration and we will have to play at the top of our game.”
