Swansea midfielder Allen was ruled out of Monday’s opener against the USA, where Gareth Bale’s late penalty earned a 1-1 draw, but manager Rob Page has since confirmed he is ready to hit the ground running again.
“The medical team has been exceptional and thanks to them he is now fit,” Page said.
“We were all worried about him as he kept breaking down. Along the way, he had to meet the goals, he met them all. If we played him in the first game and he broke down, he would be completely out.
“We could have pushed him to the US but for confidence we have to get out of the group I want him long term.”
Fellow striker Kieffer Moore is expected to start after being left on the bench in the first half against the Americans, with his introduction leading to a much improved second half performance.
Teammate Harry Wilson told reporters on Wednesday: “The game was perfect for Kieffer to come on at halftime. He gave us a platform to put him up and play him.”
“The next game could be perfect for DJ (Daniel James) or Brennan (Johnson). We’re lucky with the forwards we have.
“I would say this is a must-win game after we started with a solid point. Now we have that point on the table, we go into Friday’s game to win. Let’s hope we can do that and then focus on the game with England.”
“There is medical protocol we need to respect and there is no way to move around the protocol, so he will not play,” said coach Carlos Queiroz, who will go with back-up stopper Hossein Hosseini instead.
Wales predicted XI:
Hennessey; Mepham, Rodon, Davies; Roberts, Ramsey, Ampadu, Williams; Wilson, Bale; Moore
Iran predicted XI:
H Hosseini, Rezaeian, Pouraliganji, M Hosseini, Mohammadi, Gholizadeh, Noorollahi, Ezatolahi, Hajisafi, Azmoun, Taremi