Fernando Santos felt his players lost focus against Luxembourg as they were thinking ahead to the crucial Euro 2020 qualifier with Ukraine.
Portugal picked up their fifth win on the spin in all competitions with a straightforward 3-0 triumph in Lisbon on Friday.
Bernardo Silva gave the reigning European champions an early lead, yet the hosts had to wait until 65 minutes to score again, Cristiano Ronaldo eventually doubling their tally with an excellent chip.
Portugal never looked in any danger of conceding and added another goal in the final minute of the contest through substitute Goncalo Guedes.
As for Ukraine, they saw off Lithuania 2-0 and remain five points clear of Portugal at the summit of Group B ahead of Monday's clash between the nations, with Santos believing that fixture is a shoot-out to determine who will finish top of the table.
"It will definitely be like a final," he told Sport TV. "We had a very good first 25 minutes today, playing well and disrupting the Luxembourg team.
"From then we made the game more predictable and Luxembourg forced us to run around.
"In the second half we got in better, we made the second goal and from there the players had their heads in Ukraine - even myself, which is not normal for me. I started thinking about it and doing effort management."
Midfielder Joao Moutinho agrees with Santos' assessment that Portugal could have done more, though keeping pace with Ukraine - who have played a game more than Portugal - was the most important thing.
"We didn't score as many as we wanted," he said. "We did our best and the result is what it is.
"The game became a little bit broken and we didn't push the way we wanted. But we had a good game, we managed to control the match, create opportunities, score goals and get out of here with the victory."