The injury occurred on Saturday in London, when the Blue Bulls played against English club side Saracens in a warm-up friendly.
Van der Merwe injured his knee and pulled his hamstring in the second half and will be out for three to four weeks, team doctor Org Strauss confirmed on Monday.
The Springbok prop was the only casualty in the match and Vodacom Bulls coach Frans Ludeke should have a fairly fit squad to pick from for their first match, against the Stormers at Loftus Versfeld on February 14.
The long term injury list remains the same:
Bandise Maku (pectoral, July), Flip van der Merwe (pectoral, mid-April), Hencus van Wyk (lisfranc fracture dislocation, end of February), Pierre Schoeman (ACL rupture knee, 15 July), Adriaan Strauss (calf, 9 Feb) and Jason Jenkins (quad, 2 Feb).