Both players will definitely miss United's FA Cup match at Huddersfield Town on Saturday, though the trip to the Ramon Sanchez Pizjuan might give them the chance to make a comeback.
Rashford was absent for the 1-0 defeat to Newcastle United last time out due to a hip problem, while a thigh issue forced Herrera to miss each of their last two games.
When asked whether the pair could feature in Spain, Mourinho told a news conference: "They have a chance."
But Mourinho is trying not to give the Sevilla match too much thought ahead of the Huddersfield game, which he feels is more important.
"Let's go step by step," Mourinho said.
"The next one is the one that afterwards we will be out of the FA Cup or in a quarter-final.
"Saturday, black or white. It's not grey. Huddersfield is really important because it's a knockout match.
"We are going to face the game in the best possible way. Sevilla is half of a knockout, so not so crucial as Huddersfield, but almost."