Postecoglou admits he sees the striker as a first team prospect, so ruling out a loan departure this summer.
"Yeah we've been really happy with Will. Unfortunately he picked up an injury (hamstring) in that game against Bristol Rovers, but he should be all right after the window," said Postecoglou.
"That's why we're keeping him with us, him and Mikey are the two young players we're going to keep with the first-team and the reason we're going to keep them with the first-team is because we feel like they're going to contribute and play.
"Whether that is Europa League or Carabao Cup or the league, they're part of the first-team and if Richy (Richarlison) is out for an extended period of time, then Will will definitely be a part of it. Will and Mikey (Moore) have done really well in pre-season and we feel they'll benefit from being around the first-team."
Posetcoglou also said of new signing Wilson Odobert: "Wilson was great last week. I was wrapped to get him. He was one I was really keen on all summer.
"We had to be patient with him as the Dom Solanke deal went on a bit longer than we thought it would and we had to get that done first. He did great for us. It was his first game, as a 19-year-old, and he was exciting to watch. He will get more and more comfortable with us. I don’t mind players coming in early and us giving them exposure early as it gets it out of the way.
"You can try to protect them but when you're at a big club, get them out there and get them an understanding of what the team is like and he adapted really well."