Phil Parkinson's men head to Finland on the back of a goalless draw against Wisła Kraków at the Synerise Arena last weekend in the Reds' opening pre-season friendly in Poland.
While Wrexham returned for training a fortnight ago, United confirmed that their players would begin to report back to Carrington from Thursday July 9.
The squad will begin as a mixture of senior players and exciting Academy prospects.
With many others on duty at the World Cup this summer, the group will then grow in experience as international players start to return throughout July and early August.
Lisandro Martinez and Kobbie Mainoo are still involved with Argentina and England respectively and won't face Wrexham while Bruno Fernandes and Matheus Cunha have exited the competition but should also be absent this weekend.
Wales international Darlow was involved in friendlies with his country ahead of the finals and he now has a new club, with the veteran goalkeeper arriving at Old Trafford from rivals Leeds United on a free transfer, so he could be in line to play against the Reds.
The 35-year-old turned down the offer of a new deal at Elland Road and has instead signed a two-year contract with the option for a further 12 months with United, where he will serve as Senne Lammens’ back-up.
“I am extremely proud to sign for Manchester United,” Darlow said. “I’m joining an excellent group of goalkeepers and I’m really looking forward to all pushing each other to ensure that we maintain the highest standards, which this club demands.
“This is a really special opportunity; everyone can see what an exciting time it is for the club and I cannot wait to play my part in supporting my team mates and helping to drive the group forwards.”
Darlow’s arrival comes as Lammens’ current back-up Altay Bayindir is expected to leave United, having been linked with a return to his native Turkey. Veteran Tom Heaton, 40, signed a new one-year deal earlier this summer.
Darlow joined Leeds ahead of the 2023-24 season and made 38 appearances for the Whites, 22 of them in the Premier League.
A Leeds statement said: “Thank you Karl for your dedicated service to Leeds, we wish you and your family the best for the future.”