• Spurs striker played in Arsenal’s academy when he was eight years old • Wenger very concerned by injury situation at club before international break

Arsène Wenger has admitted he did not know whether to laugh or cry when he discovered last season that Harry Kane had been on Arsenal’s books as an eight-year-old.

The manager said he found out via the newspapers, having been unaware at the time and he promptly asked questions behind the scenes as to how Kane could have got away to join Tottenham Hotspur, who his club host in Sunday’s derby at the Emirates Stadium.

“I didn’t know, I read it in the papers,” Wenger said. “I found it quite funny, you know, and you are always a bit angry as well because I asked: ‘Why did he go?’ But at that age, boys can move here and there.”

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Kane spent a year at the Arsenal academy before they let him go and he was also rejected by Tottenham before they took him on at the age of 11. His rise through their ranks to his explosive breakthrough year last season has been well documented, and arguably the highlight was his two-goal performance in the 2-1 win over Arsenal at White Hart Lane in February.

After that game, pictures emerged of Kane as a 10-year-old with dyed red hair at Arsenal’s Invincibles title parade and he told the Observer that if he had not scored those goals, they would never have come out. A little like Wenger, Kane has been keen to forget the dalliance with Arsenal.

Wenger said he had not spoken to the 22-year-old about it but he was generous in his praise. “He is a very interesting and very strong striker,” Wenger said. “He has top qualities and I think he will make a great career.” When asked whether Kane had ever been on his transfer market radar, Wenger replied: “Once a player is at Spurs, you do not even consider him any more to be on your radar because you do not, historically, have many transfers from one club to the other.

“Sol Campbell? He was free, Campbell was out of contract,” Wenger said. “We didn’t negotiate any price.”

Kane has five goals in his last three games for Spurs and Wenger is wary of his threat, particularly with his defence having given a dreadful display in the 5-1 defeat at Bayern Munichon Wednesday. Wenger is hopeful that the centre-half Laurent Koscielny will be fit to come back after a slight hip problem – he puts his chances at 80% – and the broader topic of the squad’s injury issues preoccupied him.

Although Mikel Arteta and David Ospina are primed to return, Wenger is without seven players – Héctor Bellerín, Aaron Ramsey, Jack Wilshere, Tomas Rosicky, Danny Welbeck, Alex Oxlade-Chamberlain and Theo Walcott.

“I am very concerned, honestly, because wWe are only in November and to have so many players out, knowing we go into the Christmas period soon – I am concerned,” Wenger said. “At the moment, we have too many players out.”

Wenger is now nervous about releasing some of his players to international duty after the Tottenham game, particularly Koscielny – if he plays in the derby. “If he does not play on Sunday, I will not let him travel [with France],” Wenger said. “I have the same problem with Ospina, who might be available for Sunday. If I put him on the bench, he will go to Colombia.”

Ospina has been out with a shoulder injury that he suffered on Colombia duty during the last international break in October and it was put to Wenger that his selection for the derby stood to be influenced by the various national team call-ups.

“Honestly, no, it won’t be,” Wenger said. “You want the best players available for Sunday and you just think what happens after, happens. But it is a problem. Ospina came back injured from Colombia, he did not play for the whole period and now he travels back.”

Wenger enraged Chris Coleman, the Wales manager, with his suggestion that Ramsey’s hamstring injury, which he suffered in the home win over Bayern on 20 October, was down, in part, to him having played the 90 minutes against Andorra seven days earlier. Wales had already qualified for Euro 2016. Coleman called it a “cheap shot” but Wenger stuck to his guns.

“When a national manager has a very important game, like a qualifier, I don’t try to influence the national manager,” Wenger said. “When they play friendly games, I think: ’Why should we lose another player to lose a friendly game?’ That was why the little problem for Ramsey happened.

“Wales were qualified and they played against Andorra. I don’t see why it was especially needed to play Gareth Bale and Ramsey. They are players who played in the Champions League the week after. It is situations where the players who are most overloaded with games play.

“I just said that it would make common sense not to use Bale or Ramsey once they were qualified, and bring them on with 10 minutes to go. It would have helped the clubs and it was just an opinion. We analyse the reasons why our players get injured and you always think that, in a period like that, if a player can have one game less, it is not bad. I just felt it was one game too many.”