The Scarlets are the only unbeaten team in the GUINNESS PRO12 - sitting pretty at the top of the table with five wins from five going into the Dragons clash.
Van der Merwe has scored three tries in his first two games since returning to the GUINNESS PRO12 - including victories over both Leinster and Munster.
The winger and prop Lee will also sit out the game away at Leinster on 6 November, but will return for the European campaign that gets underway the following weekend.
"It's part of the process we've gone through with individuals," said Pivac.
"We asked DTH to do a job for us over those two weeks and he's having a good break, well-earned.
"We've got to look at the big picture when we discuss these things and hopefully it's the right thing to do at this time of the season.
"Samson's come back from injury, did really well to come back and has had a workload with Wales.
"We want to give him a couple of weeks to freshen up and we'll be working behind the scenes on his match fitness so we think the time's right there."
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