The Dutchman suffered torn muscle fibres in his hamstring in Friday's 2-0 defeat to Fortuna Dusseldorf, limping off after only half an hour.
"It didn't feel good," he said on the club website. "I had the feeling straight away that I had torn muscle fibres."
A scan confirmed the worse with Van der Vaart now a doubt for the rest of 2012, given there are a little more than four weeks left and he has been ordered to rest for three of them.
Son Heung-min will join him on the sidelines after picking up a thigh strain against Dusseldorf, w match the whole club are keen to put behind them.
"We could have taken a step forward, yet we now have to focus on the Schalke game," Germany international Dennis Aogo said.
"We start from scratch in that game."
Schalke coach Huub Stevens is sure his opposite number Thorsten Fink will find an adequate replacement for Van der Vaart.
"I doubt they will only play with 10 men," he said. "All clubs have got to deal with setbacks now and again, but Hamburg have a strong squad and they have already shown that they will be challenging for a place in Europe.
"We are certainly warned."
Stevens could welcome Ibrahim Afellay, Marco Hoger and Christoph Moritz back into his side as the Royal Blues bid to bounce back from a disappointing home draw with Eintracht Frankfurt.