Defoe struck soon after coming on in the 2-2 result.
Redknapp said: "It's difficult because I've got [Defoe] and Rafa and I can't play them both. I tried to play them both against Arsenal and it was difficult, but we were at home and I think at home I will get away with it sometimes.
"But away from home it's a problem. Rafa is not a wide player and if I play two upfront with Rafa wide against better teams then I will get overrun.
"Really I've got a choice, away from home or at home against a better team I either play Rafa or I play Jermain really."