Southampton manager Mauricio Pellegrino has blamed Tottenham's Premier League slump on the tolls of Champions League participation.
Spurs beat Real Madrid and Borussia Dortmund to top spot in their group, but are fifth in the league standings ahead of their Boxing Day clash with the Saints at Wembley.
"The main problem is that they are competing in the Champions League," said Pellegrino. "You have to spend a lot of energy, a lot of strength in this competition.
"When you are playing in European competition, it is a big change. You have to live this over a lot of years to keep the level, especially mentally. It is not easy for the players to play not just one competition, but two, or three or four."
Pellegrino also blamed heightened expectations, pointing to attacking midfielder Dele Alli as an example of a player unable to consistently overperform.
"When the expectation is really high, people can be disappointed," Pellegrino added. "Before nobody expected too much from Dele Alli and everybody talked about what a spectacular player he is.
"Now the expectations are high, people are disappointed. Always, it is like this. It is proportional.
"He is an amazing player and obviously still very young. Nobody is consistent 365 days of the year. Human beings play football, not machines. Everyone has the same trouble."