Telles will be out for the August Premier League games against Leeds, Southampton and Wolves.
Ole Gunnar Solskjaer disclosed earlier that the defender was expected to be "out for a few weeks" but a further update said the news was "worse than we hoped for".
"Unfortunately, it’s worse news on Alex Telles than we hoped for, his ankle injury is a little bit more severe than we hoped and he’ll be out for a little spell at least, the whole of August I would think."
Telles took to his Instagram page to provide an update on the injury, hoping to come back stronger.
"Nothing happens by chance, nothing comes to bring us down, but to make us evolve and mature. Thanks for everyone's messages.
"You can be sure that I'll come back stronger and even more focused. The change in me will be big. Wait for it."
Telles started United's first pre-season friendly against Derby County but picked up an injury during the club's training camp at Pennyhill Park last week.
Missing a freindly against Queen Parks Ranger on Saturday, the Brazilian left-back has been ruled out for a month.
Man United signed Telles from Portuguese side FC Porto in 2020 for a fee siad to be in the region of £15.4 million.
Solskjaer is left with two left-back options in the shape of Brandon Willaims and Luke Shaw who is still enjoying his holidays after the Euro 2020.
"That is football. We’ve got kids wanting to play of course. Brandon has done well in the left-back position," said Solskjaer.
United will face Marcelo Bielsa's Leeds United as a season opener on August 14.
