Janssen, who signed from Wolfsburg in the summer, has featured in every game this season and can play in defence or midfield.
The Netherlands international picked up a knock, however, in Sunday’s victory at Tottenham Hotspur and requires a check-up.
“We get an assessment today,” Marc Skinner told club media on Wednesday. “So I think it was actually a physical knock, so hopefully it's maybe a bit of bruising or something we can see.
“I’ll update when we find out later today.”
Skinner went on to explain the impact he feels the 30-year-old has made since her arrival, with her energy and pressing a big reason behind the fact we’ve conceded a league-low five goals all season in the WSL.