Following Sunday’s 1-1 draw with Coventry, the Black Cats boss said he still wanted to sign four or five more players before the end of the transfer window on September 1.
“Potentially, this will be the toughest period,” said Neil after the Coventry match.
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“It's hard to say for sure because I don't know what the season is going to look like, but this is the period where we're not at full strength at the moment.
“We're going to go into these first few games quite light in some areas.
“It's not through a lack of effort behind the scenes, it's because the teams we are dealing with are reluctant to let their players out at the moment and we have to bide our time.
“But we're still players light, so the likelihood is that by the end of the window the squad will look different to how it looks at the moment.
“You don't get handed your squad and told that that's your squad for the remainder of the season, because if that was the case you could get a lot of work in early doors.”
Despite the arrival of Ellis Simms on a season-long loan deal, Sunderland are still looking to strengthen their forward options while Simms’ Everton team-mate Nathan Broadhead remains on the Black Cats’ radar.