Chief of the General Staff, General Sir Peter Wall has spoken of the "shock" he felt at the moment he learned that the army was to be reduced in size.
Gen Wall said David Cameron told him about the changes - which will see the army cut from 102,000 to 82,000 personnel - last July.
He warned of a "welter of unhappiness" as up to 10% of personnel are made compulsorily redundant.
Sir Peter Wall: 'It came as a shock'
In an interview with the Financial Times, Gen Wall said: "It was very stressful at first, it came as a shock.
"It is not straightforward to do something like this when you are in the middle of an overseas campaign."
Thursday's announcement that the army is to lose 17 major units, including five infantry battalions, was met with anger by many but Gen Wall insisted that there was no truth in rumours that generals were quitting over it.
He said: "I'm confident that I've got the senior leadership of the army down to divisional and brigade level behind me on this. We've certainly given everyone the opportunity to protest."
But he added: "If you had said to me when I took over as head of the army two years ago that this was the situation I was going to have to deal with, I'd have said 'Crikey that's going to be a bit challenging'."
Gen Wall said that following the Afghanistan conflict, it was unlikely the UK would undertake a similar campaign soon and so the army would be concentrating on other tasks such as training foreign armies to better manage their national security and helping support civil authorities in times of emergency.
He told the paper: "We are creating an adaptable force with a broader repertoire of tasks."
Gen Wall, who has served in Iraq and Afghanistan, said he wants the Government to pay for new equipment and to put money into improving new barracks and facilities for the 20,000 troops returning to the UK from Germany.
He also spoke of the need to make a bigger commitment to the reserves, citing the example of the US where he said there was a "deep sense of corporate social responsibility" towards them.
As part of the changes the Territorial Army is to be doubled to 30,000, resulting in a combined force of 112,000.
The Army 2020 plan will see four infantry battalions completely disappear.
They are the 2nd Battalion the Royal Regiment of Fusiliers, the 2nd Battalion the Yorkshire Regiment (Green Howards), the 3rd Battalion the Mercian Regiment (Staffords), and the 2nd Battalion the Royal Welsh.
A fifth, the 5th Battalion the Royal Regiment of Scotland (Argyll and Sutherland Highlanders) will be reduced to a single public duties company carrying out ceremonial duties in Scotland.
The Armoured Corps will be reduced by two units with the merger of the Queen's Royal Lancers and the 9th/12th Royal Lancers and the 1st and 2nd Tank Regiments amalgamating.
There will also be reductions in the number of units in the Royal Artillery, the Royal Engineers, the Army Air Corps, the Royal Logistic Corps, the Royal Electrical and Mechanical Engineers and the Royal Military Police.
The former head of the army, General Lord Dannatt, has warned that relying so heavily on reservists was "risky".
Meanwhile shadow defence secretary Jim Murphy described the plan as a "military gamble" which would leave Britain with its smallest army since the Boer War.