Millwall's Northern Ireland and Republic of Ireland contingent can book their tickets to Russia with World Cup Play-Off victories over the international break.
Shane Ferguson, midfielder George Saville and right-back Conor McLaughlin have flown to Northern Ireland ahead of Thursday night's first-leg tie against Switzerland at Windsor Park.
Aiden O'Brien will hope for his first taste of international action with the Republic of Ireland after being named in the squad for Saturday's fixture in Denmark and the reverse leg in Dublin three days later.
Meanwhile, goalkeeper Jordan Archer has rejoined his Scotland team-mates for Thursday's friendly against the Netherlands.
"We wish them luck and hopefully they stay free from injury," Lions boss Neil Harris told millwallfc.co.uk.
"We hope they have plenty of highs while they are away and come back refreshed."
The international break will provide a chance to rest and regroup following the 1-0 defeat to Burton Albion at The Den that saw the Lions slip to 19th position on the table.
"We have to learn from that defeat," Harris added.
"We have to make sure we keep improving because we have a brilliant run of fixtures up until the New Year. We have to be better and more ruthless in the future because it is costing us points."