The 19-year-old is in the England Under-21s squad for the first time, after he was selected by Southgate for this summer’s Euro Finals.
He reported for duty this week on the back of Chelsea’s post-season trip to Australia, where Mourinho questioned his work-rate in a game with Sydney FC.
England v Belarus
A Vauxhall International
7.45pm, Thursday 11 June 2015
Oakwell, Barnsley FC
Live on BT Sport
Call 0871 2266777 for tickets
And while the U21s head coach understands Mourinho’s concerns, he has backed Loftus-Cheek to emerge even stronger from the warning.
"He’s at a club where the competition for places is as hard as any club in the country for young players coming through," said Southgate.
"They have a fantastic academy system, but the level needed to get through is higher than the rest of the league so it’s a tough challenge.
"If you’re going to get into these clubs then the demands every week from the fans and the expectation of the club is huge, and the manager is trying to embellish that point with the way he raised it after the game.
"He’s had a little dip and he’s been reminded that there’s no room for dips and I think you do tend to be tough on younger players.
"There’s a phrase, ‘be tough on talent’ and he is a talented player.
"He’s had a little dip, as young players do, and a reminder and that will be beneficial for him.
"I had a chat with Ruben about it and he knows the level that’s needed and what he’s got to do.
"He’s a determined kid, but he’s young and we keep challenging him so in a sense, it’s good because if a manager didn’t care about you he wouldn’t make that comment.
"This isn’t a character that there’s issues with, this is a lad that’s a good kid who trains well and we wouldn’t have brought him into the squad if that wasn’t the case.
"It’s a constant challenge for young players and he’s aware that he’s dipped his toe in the Premier League and it’s gone fine, but he knows that he’s got to keep proving himself."
Loftus-Cheek is the youngest member of the squad and the only one yet to be capped at U21 level.
But after being a regular for England throughout the development teams over the years, Southgate believes he is ready for the next stage.
"We hadn’t picked him up to this point because he was with the U19s, who had their own qualifiers so we were hoping they would get to their finals and we don’t want to move players up too early,” revealed the former Middlesbrough boss.
"But once they’d gone out, he became a possible for us and he’s a player that from U16 level has had a good impact with our teams.
"Sean [O’Driscoll, U19s head coach] was very pleased with what he’d done with the U19s this year and in that area of the field we’ve had a couple of players that have been carrying injuries.
"So we thought, in terms of the balance of the squad, it’d be good to have another midfielder and he’s a different profile physically to the other players that we’ve got.
"We’re excited about him and we thought it would be good for his development and for us to have a look at him and we think he can have some sort of impact at this tournament.
"Whether that happens or not, we think he’s going to be an important player for next year so it’s good to get him in the group and get an understanding of how we work."
Loftus-Cheek and his new team-mates will play Belarus in Barnsley on Thursday in their final Euro warm-up. Tickets for the game are on sale now, priced at £10 for adults and £5 for concessions. Click here to purchase online.