“When you live everyday with the lads, it gives you extra confidence,” Konstantopoulos told BBC Tees.
“Even after Charlton (a 2-0 defeat on 13 March), where everyone thought ‘that’s it, they’ve blown it’, I knew it was a matter of one win that was it and going on a good run which is what happened.
“When you train with the lads everyday that the people outside the group can’t see, you can see the togetherness, the quality and the character, I had no doubt.”
Boro missed out on escaping the Championship last season, when they lost to Norwich City in the play-off final at Wembley, having dropped from first to fourth in the final two games of the campaign.
However, that disappointment was a motivator for the Teessiders, with a core of north east players in the squad such as Ben Gibson, Stewart Downing and Grant Leadbitter desperate to restore top-flight football for the first time since 2009.
“After last year I knew the strength and character we had,” Gibson said.
“I knew the chairman would back the manager again and invest in more experienced and talented players, from minute one I think if you asked any of your players we believed we’d go up.
“There were times where people said we bottled it, we’d fell away and our moment had gone and it shows our character which you see in times of adversity.
“We came through the other side and look what we’ve achieved.”