The Toffees manager recruited Aiden McGeady, Lacina Traore and Jindrich Stanek in the transfer window and feels any more would have required "gambling" with the squad dynamic.
The Spaniard recruited winger Aiden McGeady and young goalkeeper Jindrich Stanek, as well as on-loan forward Lacina Traore, but argues that any more upheaval in the middle of the season would have meant taking a "gamble" on the dynamic of his thus-far successful squad.
The Toffees came from behind to beat Aston Villa 2-1 on Saturday and Martinez feels that the result justified his faith in his team.
"You saw the spirit we displayed out there and I didn't want to affect the dynamic of what we have got here by taking a gamble in the window," the ex-Wigan boss told reporters. "The January window is exactly that - a gamble. You go in if you need to gamble to change things around but, when you look at this squad, we don't need to.
"This dressing room is strong and I don't want to undermine that — [Saturday's] performance proves that. It was an exciting performance because it shows we can challenge in the final third of the season.
"You don't get a harder test of a team than on the back of a derby defeat, the way that it hurt everyone, and then conceding a goal to be behind like that. That sort of adversity is going to tell you an awful lot about a group of players and how you are going to finish the season.
"For me, it answered so many questions about the players and showed we are going to see a much stronger Everton."
The 40-year-old paid particular tribute to Kevin Mirallas, whose free kick provided the winner.
"There has been a change in Kevin's mentality this season," Martinez said. "He was a player helping the team, now he is a player setting standards week in, week out. He has that maturity now and has become a different footballer. He brings that star quality to be there when it is needed."