Scotland lock Richie Gray will join their tour of Japan late after Castres' qualification for the Top 14 play-offs.
They sealed a top-six finish with Sunday's 35-14 victory over Stade Francais, securing a quarter-final next weekend, when Scotland fly to Japan.
Even if Castres beat Montpellier to make the last four, Gray will join the national team after the game "no matter what", according to an SRU spokesman.
The Scots have five days on arrival to prepare for the first Test on 18 June.
They arrive in Tokyo on Sunday, before travelling on Wednesday to Nagoya, adjacent to Tokyo City, the venue for the first Test.
Castres do not play Montpellier until Sunday, 12 June at 1615 French time, and the SRU expect Gray to travel to Japan "on the next flight available". But he is not likely to arrive in Japan until late Monday or Tuesday.
The rest of the squad are gathering at Murrayfield on Monday, 6 June, for a pre-tour training camp until Thursday.
Head coach Vern Cotter will be keeping his fingers crossed that nothing befalls Gray in what will be his last game for Castres before a move to Toulouse next season.
The 26-year-old is one of only three locks, alongside younger brother Jonny and Tim Swinson, named in the 27-man squad.
The original party has already been shorn of centre Alex Dunbar, wing Tim Visser and fly-half Finn Russell, with Matt Scott, Sean Lamont and uncapped South Africa-based centre Huw Jones replacing them.
The second Test takes place in Tokyo on 25 June, with both matches broadcast live on BBC Scotland television, radio, and online.