Pompey will pay Basle an undisclosed compensation fee for Subotic, 18, who has signed a three-and-a-half-year contract at Fratton Park.
"But we still need to get some people in and it is hard. You look around and wonder who is going to loan you anybody. The only place you might get them is abroad.
"We need to strengthen out wide and up front in particular. We have just 14 outfield players here now who are capable of playing in the Premier League."
Redknapp has spent his £30million transfer budget this season and Pompey have established a place in the top 10 of the Premier League.