The Spaniard, whose team appear on course for European football next season, has a budget of more than £20m to play with, according to the Daily Mail.
"Whether we are in Europe or not is a big factor," said Martinez. "We've got money but I didn't think spending in January was the right thing to do.
"If we get in Europe I think that makes a difference of six players. I always found you can work on a squad of 21 players in the league and for domestic competitions.
"You need around 27 players plus keepers for Europe. It doesn't have to be 27 senior players, it can be young players too, but 27."