The England manager has left the duo out of his final squad before he confirms his party for Qatar later this year.
The Three Lions face Italy and Germany in the Nations League, giving their players just 180 minutes to impress their manager.
Sancho and Rashford will not get that opportunity and need to get back into the international picture with their club form.
Southgate leaves door open for Sancho and Rashford
Southgate did not do a press conference to go with the announcement of his squad, but told England’s official website: “We are very close to a World Cup and we feel, although our results were disappointing in the summer, we have picked on the basis of form and capability over a long period.
“There were certain players we wanted to keep involved this time, but that said it doesn’t mean that this is the end for some of the others who aren’t involved.
“Marcus Rashford, for example, has shown some encouraging signs recently. He is a player we know well so if he can perform at a high level in the next few weeks, he of course will be under consideration.
“Clearly, we have a number of players including Ben Chilwell, Luke Shaw, Harry Maguire and Kalvin Phillips who are not playing a lot of minutes with their club.
“It is not ideal, but we feel they have been, and can be, important players for us. It is not a perfect situation but there is still a lot of football to be played before Qatar.”
Southgate namechecked Maguire and Shaw there, who have been included in the squad despite falling out of favour under Erik ten Hag.
A third United-owned player is included in the squad, with Dean Henderson named with first-choice goalkeeper Jordan Pickford injured.
It remains to be seen if the stopper can do enough to force his way into Southgate’s plans when all his options are fit.
Maguire and Shaw may hope for some form and fitness on international duty, and it remains to be seen whether either will get a runout for United tonight.