Saracens must play their Investec Champions Cup round-of-16 match away from StoneX Stadium after they were stunned 32-24 by Castres.
It was the French who caught the eye in round one with six of their eight teams winning, including Stade Rochelais and Toulon away from home.
London Irish secured their place in the European Challenge Cup quarter-finals with a 10-try demolition of French Top 14 club Castres.
