Dr Fox, the architect of the reforms when he was in charge of the Ministry of Defence (MoD), predicted that there would be challenges with manpower throughout this decade.
Ministers want to cut the the Army by 20,000 to 82,000 while doubling the number of reservists to 30,000 but there have been reports of difficulties attracting new part-time soldiers.
Dr Fox warned that these could be exacerbated as the economy continued to recover.
In an interview he said: "When I was Secretary of State, I said we would only decrease the numbers of regulars when we had guarantees that we would be able to get the numbers - training and equipping up of the reserves - to match."
He told Total Politics magazine: "I think we need to keep a constant eye on the number of reservists, otherwise there will be a risk that we would have to put in to reverse the policy. That's a constant challenge and as the economy recovers it will become more difficult to recruit and retain.
"There will be challenges on manpower, definitely, throughout this decade, challenges in the balance ahead. We'll have to see how the employment market goes and work accordingly."
And he warned that relying on reservists instead of regulars created a whole host of other issues.
"If you're going to use reserves instead of regulars and are depending on them, you need to be able to get them when you want them, which is a change in relationships with employers. That's something that has to be managed, otherwise there might be a major disincentive for any employer to take on someone who is in the reserves. All these things will have to be ironed out."
Dr Fox, who was forced to resign over his links to a lobbyist, also said he would be a better minister after his spell on the backbenches.