Launchbury sustained a knock in England's final warm-up fixture against Italy, while head coach Eddie Jones was without Slade for all of the preparation games as the centre recovered from a knee injury.
However, the duo will be able to take part in England's World Cup opener against Tonga on September 22, meaning Jones will have 29 of his 31-man squad available for selection.
Mako Vunipola and Jack Nowell will still be missing, with both players expected to be out for at least the first two pool-stage matches.
"Mako and Jack Nowell will be back in the latter part of the group stages," forwards coach Borthwick told reporters following the squad's arrival in Japan.
"Slade is looking, very, very good. Joe Launchbury came off with a back injury at the weekend, but there's nothing concerning there.
"Joe will recover from the flight and will be good to go. While Mako and Jack are unavailable, Slade is on track."
— England Rugby (@EnglandRugby) September 10, 2019
England's travel plans were disrupted due to a typhoon, but Borthwick - who was an assistant to Jones during his stint as Japan head coach - says the squad are in fine spirits despite the delay.
"There was a bit of disruption, but people were very good around the airport. They gave us access to the lounge because they knew we'd been held up," Borthwick said.
"We stayed on the plane for a period of time and then went into the lounge.
"There was also a little delay outside at the airport so the boys started an impromptu game of cricket. It was good, but I didn't have an innings."
After facing Tonga, England - who crashed out of the 2015 World Cup at the first hurdle - will go up against the United States, Argentina and France in Pool C.
Photo Getty Images. Caption: England's Joe Launchbury sustained an injury against Italy