A stunning curled effort from Adnan Januzaj wrapped the game up early in the second half, with both teams making a host of changes from their previous games.
When asked if he was pleased with the outcome of the match, Martinez told reports (per Goal): "Honestly, I am, judging the manner of the performance," he told a news conference. "Remember, we made nine changes.
"I saw a group of players desperate to contribute and I think, in the World Cup, you can't be successful by hoping you get an easy path. We saw that in the Euros two years ago.
"We had to make big medical calls when announcing the group: now, Thomas Vermaelen looks like he's never been away, Vincent Kompany played 20 minutes in a competitive game, we've seen a team cope really well with the dead-ball threat of England, and every player, Thorgan Hazard playing a different position, Marouane Fellaini, Adnan Januzaj showing why he's in the squad, it's a big amount of positives.
"Now, we need to focus on Japan, England play Colombia, and if you think further than that it's a problem."
Martinez conceded that it may have been easier for his side to finish second in the group, but admits that his side now have to focus on the hand they have been dealt - a last 16 tie against Japan.
"From logistics, it was easier to stay in Moscow because the next game would have been in Moscow with an extra day of recovery, but the only thing we are concentrating on is playing Japan," he said.
"If you start thinking about who you could get, making predictions can be a little bit difficult. Germany are out and not many would think that would be a possibility; in the last three minutes, Spain were top of the group; and we saw how close Argentina came. This World Cup is very unpredictable and you need to look at yourself and how strong you can be."