Mauricio Pochettino admitted Tottenham were still "vulnerable" with transfer windows across Europe's other top leagues open.
Spurs signed the likes of Tanguy Ndombele, Giovani Lo Celso and Ryan Sessegnon during a strong window, which closed in the Premier League on Thursday.
However, they face losing players – including Christian Eriksen – to clubs in Spain, Italy, Germany or France, with those markets remaining open.
Pochettino, whose team host Aston Villa on Saturday, said closing the window earlier in the Premier League had left Spurs exposed.
"I think that makes Tottenham more vulnerable," he told UK newspapers.
"That's between the club and the players, and there are different personal situations that will happen until the end when the windows shut across Europe."
Eriksen and defenders Toby Alderweireld and Jan Vertonghen have entered the final years of their deals and could leave for free at the end of the season.
Departures would hurt Spurs' chances of success in 2019-20 and Pochettino is desperate to win his first trophy since taking over in 2014.
"My personal feeling, now I'm about to start my sixth season, is that a good season for us is to lift a trophy," he said.
"Carabao Cup, FA Cup, Premier League, Champions League, you know. It's not to put pressure on the team or the club but if you ask me to be successful this season it's not [just] to finish in the top four.
"For me, it's to win some title. Because if not, after last season and the [Champions League] final, the semi-final of the Carabao Cup, it's like 'come on, we are always there but we cannot touch the glory'."
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