Shiels is likely to play just off the main striker, former Ibrox star Kyle Lafferty, as Michael O’Neill’s side try to pull off the seemingly impossible to retain some hope in Group F.
Northern Ireland have one point from their opening two matches having been beaten 2-0 in Moscow by Russia and then held at home by Luxembourg 1-1 when Shiels scored his first international goal.
However, the Portuguese are smarting from a 1-0 defeat by Russia on Friday night to fall behind Fabio Capello’s side.
Ominously, Cristiano Ronaldo will reach the 100 caps milestone in the Dragao Stadium, Porto tomorrow night.
Former Rangers Roy Carroll and Steve Davis will also be in Northern Ireland’s side while David Healy is set to be on the bench.
Meanwhile, on-loan defenders Carlos Bocanegra and Dorin Goian face major qualification matches of their own tomorrow night.
Bocanegra, currently with Racing Santander in Spain, earned his 108th cap last Friday as the USA secured a last-minute 2-1 win over Antigua & Barbuda.
Now they need to avoid defeat against Guatemala in Kansas City tomorrow night to advance to the next phase of qualification for Brazil 2014.
Goian – on loan with Spezia in Italy – faces a top of the table clash as Romania take on Holland in Bucharest with both nations on maximum points from three matches.
The big centre back will recognise the officials with Scotland’s Craig Thomson in charge of the game.