Sterling was left out of Southgate's squad for next month's Euro 2024 qualifiers against Malta and North Macedonia, with the England manager saying he was not in consideration due to a hamstring problem.
Sterling has struggled for form and fitness since swapping Manchester City for Chelsea in a £47.5million deal last summer, and recently described this season as “one of the lowest points” of his career.
Chelsea are languishing in 11th place after a chaotic season in which both Thomas Tuchel and Graham Potter were sacked, but they are on the verge of naming Mauricio Pochettino as their new head coach, and Southgate has backed Sterling and the club to return to form.
“I can’t see a scenario where Chelsea aren’t in the top six next year,” Southgate said. “I think they will progress.
“It has been really difficult for all the managers this year but that will calm down over the summer. They have an outstanding squad, so they are going to have a good season and I’m sure [Sterling] will be fundamental to that.
“I would always back him to respond strongly and to have the resilience and tenacity to prove himself. He said himself earlier in the week he has not been happy with how it has been. That would be a temporary thing for sure.
“[When we spoke] I found him positive, strong, understanding what’s gone on. Difficult for everybody with all the changes this year at the club: three different managers, three different ways of training, three different ways of playing, so a lot of upheaval. Clubs have these moments.
“They have got no European football, so they will have a game a week. They will be really strong next season and I’m certain he will be a big part of that.”
England are well-stocked in Sterling's position and the 28-year-old lost his place in the XI during the mid-season World Cup, either side of being forced to return home to deal with a break-in at his home.
Southgate continued: “In the wide areas, it is probably our strongest area of competition -- and right-back is another one. But he has shown time and again for us that he has been an important player. His mental toughness stands out in why I would expect him to come up at a good level.
“It is difficult. I had to make calls during the World Cup where you’ve got to go with what you feel is right for the team. But that doesn’t mean that I don’t really appreciate the important goals he has scored for us, the big matches he has made the differences in, and the fact that he will do that again.
“He’s an important part of our group and that remains the same but I also understand as a player that it is difficult. But he is in an area of the pitch were players are playing well for their clubs and playing well when they are playing for us as well.
“The dynamic is different from maybe two to three years ago. We had four or four five players who were almost guaranteed to be in the team. We’ve got competition pretty much everywhere.”
Trent seems excited by what he’s been doing, he seems enthused by the role.
Southgate did not rule out using Liverpool's Trent Alexander-Arnold in midfield again after a series of impressive displays from the full-back in a more advance role for his club.
"I don’t think we should rule anything out," Southgate said. "I think [Trent's] ability with the ball we are all aware of, so the opportunity to get him into those areas.
"He seems excited by what he’s been doing. He seems enthused by the role. I know talking to him that has been the case. We are still in the early stages of it, so there’s a lot of work needs to happen."
Southgate also acknowledged that it would not be ideal if his first-choice goalkeeper Jordan Pickford was relegated with Everton on the final day of the Premier League season on Sunday.
"That would depend on how well he was playing," Southgate said, when asked if he would be prepared to pick Pickford in the second tier.
"It wouldn’t be ideal because clearly the level of attacks, finishing, speed of the game would be completely different so I think it would be difficult - but let’s see what Sunday brings."