Dallas and Washington were left on the substitutes’ bench for Northern Ireland’s first match, which came against Poland.
Michael O’Neill’s side put in a decent performance in that contest, but they were fairly defensive.
Northern Ireland rarely threatened during the early stages of the match, and Poland’s pressure eventually paid with Arkadiusz Milik scoring the winner.
Northern Ireland looked at their best when they showed a bit of attacking intent in the second half - with Dallas and Washington both on the field.
And Collymore said: "I hope Dallas and Washington get starts in the next game, crikey, maybe even a Lafferty/Grigg with a 4-4-2 system perhaps.
"I’d like to see them embrace a more attacking strategy in parts. I thought against the Poles their rugged and well disciplined rear guard and protection for the back 4 was admirable, but I had the feeling through the game that it was adhering to a plan, rather than the gaffer and one or two players taking responsibility at times to shake things up, swap and change positions, and try to put Poland on the back foot."
Northern Ireland’s next match comes against Ukraine, before they take on Germany in their final fixture of the group stages.
The Green and White Army are likely to have to win at least one of their remaining two fixtures if they are to have any chance of making it through to the knockout stages.