The 25-year-old Toon flop left the north east of England to join Champions League side Anderlecht on a season-long loan, where he has impressed during his 15 appearances in 2017-18.
Sels was poor at Newcastle following his move to Tyneside from Gent last summer, managing just 15 senior appearances for the club before being dropped for Karl Darlow during the 2016-17 campaign.
The lofty stopper claimed that Belgium manager Martinez urged him to leave Newcastle in the summer if he did not think he would be given enough game time, as the 2018 World Cup in Russia approaches.
"I played fifteen matches at Newcastle. That's not much," Sels said. "I could have stayed and competed, but I estimated my playing opportunities at Anderlecht higher. For the time being I am not worried, there will be enough matches.
"But it is true that I wanted to keep my World Cup opportunities cool by coming here. That was what Roberto Martínez had told me: that I definitely had to play, and that he did not see that happening at Newcastle.
"Now, even if I play at Anderlecht, it does not give me the certainty that I will be there. But by staying with Newcastle and sailing on the bank again, I certainly did not stand a chance."
Sels has kept five clean sheets in 10 league games for Anderlecht since moving back to his home country, as he looks to gain a place in Martinez's Belgium squad ahead of next summer's finals.
He faces competition from the likes of Thibaut Courtois, Simon Mignolet, Jean-Francois Gillet and Sammy Bossut, but is expected to be named third choice behind Chelsea's Courtois and Liverpool's Mignolet.
Sels is in action for Anderlecht in the Champions League on Tuesday night, as the Belgian side look to beat Celtic by three or more goals to finish third in Group B of the competition and qualify for the Europa League.