Gary Neville says the time between now and transfer deadline day is a "massive" period for Manchester United.
The window shuts on September 1, with United one of the clubs expected to be at the forefront of the major deals.
David de Gea's future at the club remains in doubt with Real Madrid keen on signing the goalkeeper but a fee has yet to be agreed between the two clubs, while United have been continuously linked with a move for a new centre-back.
Neville thinks that Louis van Gaal and his board have an important two weeks ahead.
"It's a massive two weeks for Manchester United in terms of holding on to players and trying to strengthen where they can," Neville said.
"They could find themselves in a position where they've only got one centre-forward and potentially not the best goalkeeper if De Gea moves to Real Madrid.
Balague: De Gea deal will happen
Guillem Balague expects David de Gea to join Real Madrid - but the deal may not be done until late.
"They could also have a problem at centre-back if they get an injury - Phil Jones is out at the moment - so they could need another centre-back. That's three main areas of the pitch.
"Centre-midfield was the position where United needed to improve in the last three or four years and they are actually OK in there now. But now they look weak up front with just Wayne Rooney and Javier Hernandez. At the back too, if they get another injury, they would look weak there."
It's a massive two weeks for Manchester United in terms of holding on to players and trying to strengthen where they can.
Makeshift centre-half Daley Blind and Chris Smalling have formed a solid partnership in United's first two games of the season, keeping two clean sheets in wins over Tottenham and Aston Villa.
However, United have been making moves for defenders in this window, including a failed bid of £28.6m for Real Madrid defender Sergio Ramos in July, according to Sky sources.
Neville has been impressed with Blind's performances but feels that Van Gaal may have his eyes on a long-term solution in the market.
He said: "Everyone thought they were going to sign a Ramos or an (Nicolas) Otamendi - those types of players - and that a big player would have been signed in the centre-back position.
"I don't think anyone would have foreseen Daley Blind playing centre-back. I gave him man-of-the-match on Friday night [against Aston Villa] but I think it's more of a stop-gap rather than a long-term solution. He knows what Van Gaal wants."
What do Man Utd need?
Another forward? Centre-back cover? Where do Manchester United still need to strengthen?