Whittaker picked up an Achilles injury in Tuesday’s 2-1 friendly defeat against Hamburg.
Whittaker told the club’s official website: “I hope it’s not problematic. Just as I was breaking through the boy challenged me from behind and I tried to ride the tackle a bit.
“He caught me on the Achilles though and it just tightened up in the next few minutes, so I just decided to come off and not make it any worse.
“The gaffer had planned to take me off after an hour and it was 52 minutes or so, so there was no point in staying on for those extra 10 minutes.
“The physios are pretty optimistic and I think I’ve just got a kick so hopefully with a couple of days that can settle down and I’ll be able to play as normal.”
