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.
Speaking following the Aviva Stadium encounter, Gatland said: “We’ll talk to the medics and see how he feels over the next few days.
“He will be assessed over the next few days. We will see how he comes out of the next couple of days.”
Asked if he felt that Patchell deserved some luck following last season’s troubles – he was concussed on two separate occasions and suffered a hamstring injury – Gatland added: “Yes he does. He’s gone a little bit low where he has probably got that knock.
“The ironic thing with World Rugby is they are keeping the tackles down, but most concussions come from the tackler going low and hitting knees and hips.
“It’s one aspect we need to continue to work on for the defensive player as well.”
World Rugby’s guidance document on recurrent or multiple concussions states: “Players with a history of two or more concussions within the past year are at greater risk of further brain injury and slower recovery, and should seek medical attention from practitioners experienced in concussion management before return to play.
“In addition, a history of multiple concussions or players with unusual presentations or prolonged recovery should be assessed and managed by health care providers with experience in sports-related concussions.”
Wales’ opening World Cup game is against Georgia on September 23, prior to which they have a week-long training camp in the southern Japanese city of Kitakyushu.
Patchell’s latest concussion came as Wales suffered a third defeat of their four-game preparation schedule.
Reflecting on the past month, Gatland said: “We’ve tried to go out there and be positive in the way we play. I am generally pretty pleased.
“You hold a little bit back and you work on a few things. That’s important.
“These are warm-up games, not a Six Nations game, and we’ve got these under our belt and tried some combinations.”
Wales hooker Elliot Dee added: “We have played some good rugby over the last 18 months, and we are really excited to get out there to Japan and show what we can do.
“We have talked about it for a long time now. It’s been a brilliant journey so far, and we have got to go out there and perform now.
“This squad has shown what it can do over the last two years. Everyone works incredibly hard, and we are looking to work as hard as we can and get as far as we can.”