Dummett lasted just seven minutes before succumbing to a hamstring problem, whilst new recruit Lejeune suffered ankle ligament damage in a challenge with England international Harry Kane.
In a club statement released on Tuesday, Newcastle confirmed that both players were set for a spell on the sidelines.
"Both players are expected to be out for weeks, rather than months," it read.
The news will come as a blow to Newcastle manager Rafa Benitez who is having problems strengthening his squad this summer.
The Spaniard said after Sunday's 2-0 defeat that Kane should have been sent off for his challenge on Lejeune.