'It is hard to understand for people in football that Tottenham didn't sign or didn't sell players but sometimes in football, you need to behave different'.
Pochettino highlighted the cost of Tottenham's new stadium, which will open this season, arguing that while it is a big investment, Brexit made it more expensive than the figures let on.
United boss Jose Mourinho has been interested in signing Alderweireld from Tottenham since last summer and the 29-year-old was reportedly ready to reject all other clubs in order to join the Red Devils.
Pochettino added: "What the club is doing is showing it is so courageous".
"We have a very good squad, we have very good players and it is not easy to add players that improve that", Pochettino said. "We finish the lodge at the training ground".
Speaking about any possible incomings, the Tottenham manager said in a press conference: "I think it's so hard now".
"Building a stadium that is almost £1 billion". I am so proud of them because they were very focused on trying to arrive in a very good condition. If all the players today are thinking about a collective target, we can be competitive. If we have all the squad thinking as individuals rather than as a collective, you can not achieve what you want and can not succeed.
Toby Alderweireld, Danny Rose and Mousa Dembele were all expected to leave this summer, and they still could if they are willing to move overseas.
The Scotland global was a key man during a loan spell at Villa Park last season, but as the Villans hit financial difficulty early this summer, they were unable to put anything in place that allowed them to keep the playmaker.
But Pochettino has warned that he will demand full commitment from anyone who is still at the club after the continental window closes.