Riis scored the opening goal in the 2-1 victory but felt a pain in his hamstring as he got up off the ground to celebrate.
He was substituted a couple of minutes later with Lowe taking no chances with his fitness.
If the Good Friday game at Deepdale comes too early for the Danish striker, Lowe expects to have Ched Evans back in contention.
Evans resumed training this week after missing the last two games with a toe injury sustained in the defeat at Derby County.
Speaking at Euxton on Wednesday morning to look ahead to Millwall’s visit, Lowe said: “We’ll have to see with Emil, he won’t be training with us today.
"I think he’s had his scan results and I haven’t heard anything in terms of it being bad, so we’re hoping it’s not too bad.
"Friday? We’ll monitor him over the next couple of days and take a view.
"Emil will get out to do some jogging first and we’ll take things from there. He is probably 50/50 for Millwall.”
To add insult to injury, Riis’ goal against QPR was put out as an own goal by the company who supply statistics to the media.
However, North End have registered it as Riis’ goal and club sources say the referee’s report of the game had Riis down as the scorer.
It takes him on to 18 for the season, 14 in the Championship and four in the Carabao Cup.
Updating the position on Evans, Lowe said: “Ched is fine, he has trained for the last couple of days. He’s just had an issue with his toe.
"He has been out on the grass and that is a good sign."
Brad Potts and Josh Earl will be back in contention after a stomach bug ruled them out of the Millwall match.
It was a bug which had laid low goalkeeper Daniel Iversen ahead of the Blackpool game but he recovered to play.
Lowe said: “Brad and Josh are fine.
"We’ve no new injury worries, there’s just Ryan Ledson and Tom Barkhuizen out. Other than that we are okay.”