Former Scarlets favourite Lamont suffered ligament damage in his right knee during the 19-17 reverse at Murrayfield, while Montpellier back rower Beattie damaged his right ankle early on in Edinburgh.
Beattie lasted just 16 minutes of a game that Scotland dominated in most areas apart from the final scoreboard and Lamont limped off after only half an hour.
The news is a blow to Scotland head coach Scott Johnson given that Lamont is his country's third-highest appearance maker with 86 caps to his name across a decade of Test rugby and Beattie has bags of experience in the PRO12 and Top14.
Glasgow's Ryan Wilson and Max Evans of Castres were the respective replacements for Beattie and Lamont against the French and now appear to be the early favourites to start against Wales.
There could be more bad news for ex-Wales caretaker boss Johnson and co as well after British & Irish Lions prop Ryan Grant picked up a hip/abdomen problem and will face a fitness test later in the week to determine his availability for Saturday.