The Scarlets number 10 suffered a third concussion in 12 months during Wales’ 19-10 defeat against Ireland in Dublin.
He went off midway through the first half following an attempted tackle on Ireland back-row forward CJ Stander.
It was Patchell’s first Test start since June last year when he starred with a 20-point haul against Argentina in Santa Fe.
He is one of two fly-halves in Wales head coach Warren Gatland’s 31-man squad for Japan, having edged out Jarrod Evans to secure a place alongside Dan Biggar.
