Roy Hodgson is worried Danny Welbeck will not be ready for Euro 2016 and that England might have to plan without him after revealing the striker’s knee injury could be considerably worse than Arsenal have stated publicly.

Hodgson’s information is that Welbeck will be out for another six months, as opposed to Arsenal’s view that it will be a three-month absence, and the England manager said he has had to make contingency plans in case the former Manchester United player is not ready.

England sealed their place in next summer’s competition by beating San Marino 6-0 on Saturday and Hodgson also revealed he had made a personal intervention to try to persuade Jack Grealish to break into the squad rather than going to the tournament with Republic of Ireland.

Hodgson has been waiting all summer for Grealish to decide and said he would like the 19-year-old to announce his plans before next month’s games against Estonia and Lithuania. “I’m not 100% certain of what he is waiting for,” Hodgson said. “As far as I am concerned I would like him to come out and say: ‘I’ve thought about it, I’ve got the two possibilities and I’m opting for England’ and then he will be available for selection.”

England’s early qualification means Hodgson will experiment with some younger players in those October fixtures, the idea being to create a pool of around 30 players so there is a “Plan B” in case of drop-outs.

Hodgson is mindful of Daniel Sturridge’s injury record, as well as Jack Wilshere’s recurring ankle problems, and said he is increasingly concerned by the prognosis from his medical staff about Welbeck’s likely return, confirming he does not expect him to be back playing until March.

“Welbeck bothers me now,” Hodgson said. “He has already been out for me since March so that’s over four months already. And now I hear it’s another six months, so I can only hope that Danny recovers a bit quicker and hits the ground running.

“Daniel is a little bit the same because it’s getting on for a year now. They are always in my thoughts because I think they’re very good players, and they did extremely well when they were playing in the national team, but I’ve got to make certain there are a few Jamie Vardys and Harry Kanes and Theo Walcotts because we can’t keep going around talking about the ones who aren’t there.

“What we’re trying to develop is a bigger pool of players, to get that up to 30, where we can say: ‘Whatever happens, as long as we can get 23 of this 30, we’ll be OK.’ Then you would have to be very unlucky if two or three in the same position went down. We don’t need more than 30.”

Hodgson is planning to bring Raheem Sterling, Michael Carrick, Gary Cahill and Chris Smalling back into his team for Tuesday night’s game against Switzerland, when Wayne Rooney will have another opportunity to reach the 50-goal mark that would elevate him to being England’s record scorer. Several players, including Rooney, may then be rested for the October qualifiers before England play friendlies against Spain and France in November.

Grealish, who has started the season at Aston Villa with an injury, was understood to be leaning towards choosing England earlier in the summer. However, Hodgson is still unsure about the teenager’s intentions and made it clear he would not be accepting an invitation to meet the player’s advisers.

“What I don’t do is speak with agents. I’m more than happy to speak to the boy. I’m more than happy to speak to his manager and more than happy to speak to his parents. But as England manager, I don’t speak to agents. Maybe the FA will have to, if the boy insists, but I’m not prepared to do that. I think it’s inappropriate, very inappropriate.”

On Grealish, Hodgson added: “We’ve spoken to him and what we’ve said, quite simply, is that we think he is a very talented footballer. We think he could have a very good international career and we would be more than happy if he chose that career to be England. I’m still hoping that’s what he will do but at the same time because of his family [grandparents] he has the opportunity to play for Ireland.

“I made it clear to him: ‘It’s got to be your decision, it’s got to be something you really want to do and that feels right because I certainly don’t want to put you under any undue pressure and I’m not going to resort to artificial means to get you with us and stop you playing elsewhere. I want you to play for England because you want to play for England.’

“Did I expect it [his decision] earlier? I don’t know. It would be nice if he says yes and then he would have a year to see if he could knock one or two of these players off their perch. That would be pleasant but there is no time limit.”