The 28-year-old Ivory Coast international, who joined West Ham for a then club record £45m in 2019, had treatment which involved chemotherapy and two lots of surgery.

He returned to training last month and made his Dortmund debut as a second-half substitute against Augsburg on 22 January. On 4 February, World Cancer Day, Haller scored his first goal for his new club against Freiburg.

Before the Champions League last-16 first-leg encounter with Chelsea, Haller sat down with BBC Sport to talk about the impact of his cancer diagnosis.

'No time to be emotional'

Haller had to clarify what he was being told - but even when the word cancer was used, it did not scare him.

"The first day I didn't know it was cancer," he says. "They just said 'tumour'. But I am not medical. I didn't know exactly what it meant. I just asked the doctor. 'It is cancer, eh?' He said 'Yes'. So I said, 'Well just say it'.

"Of course, you realise it is something really serious that is happening, that a lot of things can change. But the urologist helped me not to be scared. He said I could heal well. I took all his words for granted."

Haller is a father, a brother and a son. He is also a husband. His first call was to wife Priscilla, who at the time was on holiday with the couple's three children.

"There was no time to be emotional," he recalls. "She had the kids and I had some other appointments. I had to be straight. 'Please sit down. I need to tell you something serious.' It had to be quick so she could process.

"Unfortunately that is the position when you are married to someone. You need to share everything. You can't just avoid a few parts of the problem."

'My kids saw my face changing'

Haller was not afraid to make his journey public. He posted pictures from his hospital bed on social media. Another appeared of him bald as his course of chemotherapy started to have an effect.

"The pictures on social media reflect myself quite well," says Haller. "If I was alone, just with my wife, probably it would be different. But as soon as you have kids, you don't have time to think bad because they need you. No matter what has happened, you need to show them everything is fine.

"Of course you have some bad moments but I don't think that was the lowest part. Even if emotionally it wasn't easy because I was sick, all the attention was turned to me, not to other people, not to people from my family who suffer as well. No-one cares about them, which is a big problem.

"For them it was really complicated. They saw me changing - just my face. This was the most difficult part to handle. You feel more stressed and more worried. It makes you more careful about what we are saying and how we act between each other."

The football world united around Haller, who was overwhelmed by the supportive messages he received. One came from his old boss at Ajax, Erik ten Hag. Never once did Haller doubt he would return. He did feel bad for Dortmund, though.

"I was thinking 'they have bought a player and after two weeks I can't play for I don't know how long'," he says. "What a bad deal. Of course I wanted to come back and give back the support they gave me. This was really important. They gave to me before I could give something to them. I was sick and they gave me support. But I always knew that I would make it. A lot of it was mental and the second surgery made it a little bit more difficult, but I knew I would come back."

Haller is relishing being back in the "special" atmosphere of the dressing room. Scoring, for him, is the best feeling in the world.

"There is nothing more important than you in this moment," he says. "Everyone is looking at you. Everyone is screaming. You are on a cloud."

'We need to check - don't be ashamed'

Now Haller is looking forward with positivity. The cancer does not draw out negativity. Nor does his ill-fated spell in the Premier League at West Ham, the one time in his career when scoring goals became a problem.

"I have nothing to prove," says Haller. "Not everyone can have all the information about why a player doesn't play well, and no-one cares anyway. You are on the pitch and you need to perform.

"I didn't show the best version of myself at West Ham and it is a shame. It could have been better but this is the past.

"When you focus on the things that didn't work instead of thinking about the good things you have in front of you, you are wasting time. Every game is special in the Champions League."

Haller closes with a message. If he, a super-fit professional athlete at the height of his career, can develop a tumour, anyone can.

"The most important thing is not to think it can only happen to other people," he says. "I was one of them seven months ago. I was fit and I was doing well. In the space of three months the level I had of this tumour was really high.

"We need to check. You cannot be ashamed. We are not checking only for ourselves but also for our family and friends and those around us. It can save a lot of lives."