Guus Hiddink's side take on Mexico in a friendly in Amsterdam on Wednesday, before hosting Latvia at the same venue in their fourth Euro 2016 Group A qualifier four days later.
They will be without Ajax midfielder Klaassen, after it was revealed on Monday the 21-year-old had sustained a groin problem.
Klaassen, who made his only senior international appearance to date in a 2-0 friendly defeat to France in March, has been replaced by QPR's Fer.
Fer was part of Louis van Gaal's squad at the World Cup in Brazil, and featured as a substitute in both of the previous two qualifiers against Kazakhstan and Iceland.
After finishing third in Brazil, Netherlands have made a stuttering start to their Euro 2016 qualifying campaign, with just three points to their name from the opening three matches.