The winger played under O'Neill as Ireland stunned Italy and reached the last 16 in France, but admits the veteran manager's methods did not sit comfortably with every member of the squad.

O'Neill has come under scrutiny in recent months over a perceived lack of tactical identity, and McLean insists detailed game plans were never the cornerstone of his former boss' philosophy.

Current Celtic captain Callum McGregor recently criticised the "basketball" nature of his side's performances, highlighting the use of strikers in wide positions as one of the reasons for the team's chaotic displays.

And McLean believes younger players entering the Ireland setup ahead of Euro 2016 struggled to adapt to O'Neill's old-school management style.

Writing in his autobiography The Walls, McLean said: "The problem was the new profile of the squad. The Euro 2016 team had an old-school core. It was a team of men.

"Martin wasn't big on tactics. He would say the game hadn't changed as much as people thought. It was still 11 versus 11, and he trusted good footballers to sort problems out on the pitch.

"But we had an influx of mainly Championship players into the group. Younger lads who had come through in an environment where they were heavily coached and had everything mapped out for them. Tactics, tactics, tactics.

"They couldn't understand Martin's way of doing things. It was like: 'What's this? Why aren't we doing more?'

"They also weren't as capable of handling criticism either."