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.
|5/1||Everton are 5/1 with William Hill to finish in the Premier League top four|
"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 enthused. "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."