Field Marshal Lord Bramall said the armed forces were having to make the best of cuts driven by financial considerations.
Defence Secretary Philip Hammond says he is very confident the Army can reach a target of 30,000 reservists by 2018.
The regular Army is being cut from 102,000 to 82,000 while the Army Reserve - formerly the Territorial Army - is to grow from 19,000.
A leaked Ministry of Defence memo last month warned "disappointing" numbers of part-time soldiers are being recruited.
Lord Bramall led the Army as Chief of General Staff in the Falklands War and was later Chief of Defence Staff. He said there was "a terrible question mark" over finding enough reservists.
"If you are not going to get the reserves then you should not run down frontline troops as quickly as you intended," he said. "The logical thing is you do not run them down until you achieve the build-up of reserves, but finance is still driving the agenda."