All sides at 1st XV level involved in games that Durham Referees Society appoint to should have 1 front row replacement so at the 1st time when there is an injury to a front row he may be directly replaced and the scrums would remain contested.
If the reason the front row player leaves the field is due to a red or yellow card then a front row replacement can come onto the field (including one that has been replaced for tactical reasons) but another player must leave the field so that the side retains the sanction of having a man off the field due to the card (red or yellow)
The scrummages at this point would still remain contested as there are 3 front row players on the field

Note teams in lower level games may not always be able to have a Front Row replacement

If a front row player player has already been replaced due to injury or a card then when another front row is injured or carded then the injured or carded player must leave the field of play and cannot be replaced hence they have “a man off”,  This assumes there is not a 2nd front row replacement available. There is an added requirement that the uncontested scrummages must contain 8 players on each side. Also remember that although the scrummages are uncontested, both sides must pack down correctly ie players are not allowed to be loosely bound so they can fly off

https://www.englandrugby.com/mm/Document/MyRugby/Referee/01/30/47/58/Front_Row_Logic_Tree_1415_Level_5_Neutral.pdf

Some people on the sidelines are clearly getting confused by the laws that apply to games where there are 23 PLAYERS
The only occasions where a team would lose an extra player due to uncontested scrums resulting from a card are competitions which have 23 player squads, e.g. Premiership and World Rugby Internationals.

It will not apply to any RFU leagues below Premiership. There may be different regulations for Cups, particularly County Cups as they often set their own regulations and are sometimes slightly different to RFU’s regulations. This does not apply to Durham County Cup competitions

It’s all covered in Regulation 13.5: https://www.englandrugby.com/mm/Document/General/General/01/32/82/66/RFURegulation13_English.pdf
Appendix 2:
https://www.englandrugby.com/mm/Document/General/General/01/32/72/43/RFURegulation13Appendix2_English.pdf