Harry Redknapp's side could lose their way in the battle for a UEFA cup place and Distin said: "I think we need to invest in players.
"Clubs around us are going to do it. If we want to finish in the top six or seven we are going to have to do it too."
David James and Sol Campbell have already expressed similar sentiments and Distin added in the Portsmouth News: "We do not have enough players. We need to get some in. If we have injuries or suspensions we are in trouble.
"There were players playing at Sunderland who had not played one game this season.
"It has been a good season and we have got some good players, but to be honest I was already expecting some to be here by now.
"The more we are going to wait, the more difficult it is going to be to get some players, and harder for us to have a reaction.
"If you want to push for Europe you have to invest in every transfer window. Time will tell if we lose what we have achieved this season. We have to sort it out otherwise we are starting to drop off."