Law clarification 5 from 2015 covers this.
[LAWS]Clarification in Law by the Designated Members of Rugby Committee
Clarification 5 - 2015
Union Joel Jutge – High Performance Match Official Manager Fifteens
Law Reference 3
Date August 14, 2015
Request
1. Assuming a team only has two players in the starting 23 who can play tight head
(TH). If the starting TH of that team is injured and replaced by the replacement TH
and the replacement TH is subsequently temporarily replaced (either through blood or
a HIA), can that team bring on a replacement, even though when the next scrum is
awarded, that team will not be able to provide a suitably trained TH?
2. If that team is allowed to bring on a replacement, what should happen at the next
scrum?
3. If that team is allowed to bring on a replacement, what happens if that temporary
replacement becomes permanent?
Clarification of the Designated Members of the Rugby Committee
In specific answer to the question:
1. Yes the team can bring on a replacement
2. Uncontested scrums with 15 players
3. If the temporary replacement becomes permanent then the game continues with
uncontested scrums but with 14 players
For the case of completeness the following may help:
Definition from Law book:
Replacement. A player who replaces an injured team-mate.
Substitute. A player who replaces a team-mate for tactical reasons.
Example 1: 3 is injured and replaced (note definition above) by 18 (the declared TH cover). If
18 gets then gets injured the team goes to 14 players and uncontested scrums. (Law 3.5 h, m
and t )
Example 2: 3 is substituted (note definition above) by 18 (the declared TH cover). If 18 gets
then gets injured then 3 can return and the game continues with contested scrums (Law 3.5 r
and s and 3.13)
Example 3: 3 is injured and replaced by 18 (the declared TH cover). If 18 gets then gets
temporarily replaced for Blood (Law 3.10) or HIA (Law 3.11) the team can continue with 15
players but scrums become uncontested until 18 returns. If 18 does not return then the team
goes to 14 players and uncontested scrums. (Law 3.5 h, m and t)
Example 4: 3 is substituted by 18 (the declared TH cover). If 18 gets then gets temporarily
replaced for (Law 3.10) or HIA (Law 3.11) then 3 can return and the game continues with
contested scrums. (Law 3.13)
In order to facilitate this process and apply Law 3.5(g) then teams should declare prior to the
match (on the competition team sheet) exactly which front row positions that players are
suitably trained and experienced to play in. These players can be nominated to play in more
than one front row position.[/LAWS]