José Mourinho believes Anthony Martial’s impressive start to the season is because the Manchester United forward is far happier than he was last term.

The 21-year-old has scored four times in seven appearances, half of his total of eight last season. Martial has managed the tally despite starting only three times.

“I see a great improvement in the person, in the mood, in the face, in the body language – if you want just a word, in the happiness,” said Mourinho of Martial. “He’s a happy guy, he’s working extremely well. He starts matches, he tries to do well.

“[If] he goes from the bench, even if it is for 10 minutes like in the last [league] match at home, he tries to enjoy and try to give something in these extra minutes. I am very pleased with his attitude overall and then it’s easier to play well.”

Of Martial’s lack of regular starts, Mourinho said: “It’s hard for him but it’s hard for many others. So many players are in good moments, playing well. It’s hard for all of them. When I see the match last Wednesday [the 4-1 Carabao Cup win over Burton Albion]: the attacking players – Martial, Marcus Rashford, Juan Mata, Jesse Lingard – who deserves to be on the bench?

“No one. Nobody deserves to be on the bench. But they are four, plus Romelu Lukaku and Henrikh Mkhitaryan is six. I cannot play with the six.”

Luke Shaw, meanwhile, was a second-half replacement against Burton, the left-back’s first action since his ankle injury picked up at the end of April. But the manager said he faces a battle to regain a starting berth. “He has to work. Just that,” Mourinho said. “You saw the game. You saw the same as I see. Are you asking if he plays [at Southampton]? No, he isn’t. He has to work. He has to improve. Look, he doesn’t play for six months. I’m not expecting him to arrive on Wednesday and be man of the match or to be running up and down for 45 minutes.

“It’s a process. He doesn’t play for a long time. You have other players in the Premier League who are not starting matches and their managers just say they’re not in the best conditions. And we’re speaking about some of the best players in the league. I heard last week the Chelsea manager saying that. The Arsenal manager said that. The players are not in their ideas, they’re not in the best form after important injuries.

“Luke had surgery and a long time without playing. I cannot expect him to be back and strong, strong, strong in his work.”

After a small section of United fans sang about the size of Romelu Lukaku’s penis they were described as racist by Kick It Out, the anti-discrimination group. The striker subsequently requested that they stop singing it, yet asked about the issue Mourinho said: “Don’t ask me these questions. I want the fans to support the football and to support the players. That’s what I want.”

United travel to St Mary’s with Southampton in ninth position with eight points, five behind Mourinho’s team who are joint-top with Manchester City. Of the south coast side the Portuguese said: “Southampton is the kind of club that are very stable in the way they play and in the way they see football, the qualities of their players. I think it’s a good team to be a manager of. Honestly, I think it’s a very good club to be manager of. And I see the match as a very difficult match.”