Mount has missed his side's last two matches through injury, while James was absent against Juventus on Wednesday night due to a problem that he picked up against Manchester City last weekend.
Both players have been named in the latest England squad, though, and Southgate has said that the duo are likely to be involved in Chelsea's match with the Saints on Saturday.
"Clearly they are still to appear for their clubs so there is an element of the unknown. Our understanding is that Mason and Reece have a good chance for the weekend [against Southampton]," Southgate told reporters.
"We'll have to track that. We don't play until next Saturday so there is a bit of time and we will know more as the week progresses."
Chelsea will be looking to bounce back from successive defeats against Ralph Hasenhuttl's side this weekend, having lost to Man City and Juventus in the league and Champions League respectively. body check tags ::