Spurs parted with £30m to sign the midfielder from Newcastle United, but the 27-year-old has started just four Premier League matches and spent three games serving a suspension after elbowing Bournemouth's Harry Arter in October.
Sissoko was not included in the matchday squad for the weekend's 2-1 defeat to Chelsea at Stamford Bridge, and now Pochettino has claimed that the midfielder needs to show more on the pitch and in training to get back in the team.
"Football is not about money," The Guardian quotes Pochettino as saying. "It is about players being better, and that they show on the training ground that they are better than another teammate and that they deserve to be involved or not. In my opinion, I need to take the decisions and I believe that the players that started in the game [on Saturday] and the players on the bench deserved to be involved in the game. There's nothing else to [read into] it.
"It's not frustrating for me. It's about football. We can see in football different things that happen. You sign a player and then you expect something, and you don't find what you expected.
"If another deserved to be involved, like maybe [Georges-Kevin] Nkoudou or Josh Onomah, or [Vincent] Janssen, then why not? Just because we pay money [for someone], do they deserve to play? Sissoko needs to work hard and to show in future that he deserves to be on the team."
Tottenham sit fifth in the Premier League table, seven points adrift of leaders Chelsea after 13 games.