Winger Holmes has yet to start in the league for Pompey since joining from Barnet in the summer, with Andy Barcham and Jed Wallace currently filling the wide positions.
But Whittingham says the 26-year-old's chance will come.
"Barch and Jed are doing well but I know he's dead keen to get in," said Whittingham.
"When people see him for a couple of games they will know we have three players out wide who can attack well.
'"There's no real reason why he's been held back. There's three players for two positions so someone has to miss out."