Kevin De Bruyne believes the mental and physical toll of chasing the quadruple could make the feat nearly impossible for Manchester City.
City moved a step closer to an unprecedented clean sweep of four trophies by reaching the FA Cup final with a narrow 1-0 victory over Brighton at Wembley on Saturday.
Yet manager Pep Guardiola described the quad challenge as almost impossible after the game and De Bruyne agrees that the congested fixture list presents a tough obstacle.
The Belgium playmaker said: "It is nearly impossible. I don't think it is impossible to win every game, (looking) game by game, but if you see the fixtures coming up - I think every game we play now we are going to be at a disadvantage physically and mentally because we will have a game before the other teams, the other teams will have a rest.
"The Champions League is the same. So, it is difficult."
City are now staying in London ahead of their next testing task, tomorrow's Champions League quarter-final first leg at Tottenham.
To underline his point, De Bruyne noted that Spurs - who City also face in the Premier League this month - have had a free weekend ahead of the clash.
The 27-year-old said: "Every game is difficult. We just now have to prepare for Tuesday.
"We know Tottenham played their last game on Wednesday so they will be more fresh than us. We have to prepare ourselves mentally and physically to be ready."
The game will be Spurs' second at their new stadium, the move to which has given the club a lift.
But De Bruyne said: "The stadium doesn't matter. They are going to be excited because it is a Champions League quarter-final.
"Spurs are a very good team. Everybody knows how they play. They will not change. It is going to be exciting, it is going to be tough because we play them three times in 10 days."
While City were involved in FA Cup action over the weekend, title rivals Liverpool returned to the top of the table with a hard-fought win over Southampton. City are still favourites because of their game in hand, but De Bruyne does not think the Reds will allow any margin for error.
He said: "Probably they will (win their games). We just have to look at ourselves and try to win game by game. We will see in the end what happens.
"No matter who is champion or second, we've had a great season. What can you say if you get 92 or 95 points and in the end you are second? Normally that's historic in the Premier League, to get these points. Imagine being second with it."
De Bruyne provided the one genuine moment of quality in a drab game against Brighton, curling in a brilliant fourth-minute cross for Gabriel Jesus to head the only goal. The Seagulls applied pressure in the second half but City comfortably closed out the win.
"We know how Brighton play," De Bruyne said. "They played well and it was difficult for us but, except for one corner, I don't think they threatened too much."
Meanwhile, Benjamin Mendy insists he is no troublemaker and vowed to stick to the rulebook as the City left-back looks to make up for lost time.
The 24-year-old has endured a topsy-turvy time since arriving at the Etihad Stadium in 2017.
Mendy's first season with City was brought to an abrupt halt by a cruciate ligament injury and, having helped France win the World Cup, on Saturday he made his first start since undergoing meniscus surgery in November.
Disciplined last September for reporting late to have treatment on a foot injury after attending a boxing event, Mendy was this week subject of reports that he was in a nightclub in the early hours having not been included in the squad for Fulham later that same day.
"I feel so good, and I am very happy," the City defender said.
"The team-mates and the staff have been really good.
"I am not a doctor. I follow the rules. The excuse is not one.
"This injury (following an attempted comeback in January)... when I came back sometimes the swelling stayed.
"But now it's good, we can forget the past and think about the future.
"I feel really fit and I want to give everything I can to help my team. I have missed a lot of games, but my team-mates have been incredible," Mendy said.
"So we need to trust and to believe it for going on the end with a lot of trophies.
"We will have a good rest and we will see after when we have seen the manager and follow the rules.
"We are happy we want to win as many trophies as possible. So we try to play good, to give everything for the fans. That is what we did today."