Hammond unveiled the Green Paper Future Reserves 2020, which included plans to increase the number of reservists from 15,000 to 30,000 soon after regular army numbers have been slashed by 20,000. Under the proposals, now open for consultation, the current Territorial Army (TA) would in future be known as the Army Reserves.
The proposals, which include allowing reserve soldiers to take up to 40 days a year, have been criticised by business groups.
Jim Murphy, Labour's shadow defence secretary, said: "Without business on side, increased reservist numbers will falter.
"Having staff away for 40 days a year and one year in five is a significant ask which large companies may be able to absorb but which will be tough for SMEs.
"At a difficult time for many companies, employers must be given the support they need when their workers serve on reserve duty.
The announcement also mean that employees who work as army reserve troops could be in action for up to a year in every five.
Mike Cherry, national policy chairman of the Federation of Small Businesses, said: "We have some concerns about the extended time a staff member could be away from the business - for the smallest of firms this could make it more difficult to be supportive of their reservists."
Cherry added: "There also needs to be a clear line of responsibility for who is accountable if the reservist is injured while on duty."
John Cridland, CBI Director-General, said: "This is the biggest change for reserve soldiers since the Second World War and the CBI stands ready to help the make these reforms work.
"It will require a new partnership between business and the Ministry of Defence, and that will need to be built.
"With reservists in the future likely to be absent from the workplace for longer periods of time and more often, managing this new system will be akin to dealing with maternity leave."
Cridland added: "Such a significant change for employers means that the MoD needs to actively engage with and support companies to make this new arrangement work."