The England midfielder's season is likely to be over as he faces ACL surgery.
Asked about the need to go to the market for a replacement, Frank said after their preseason friendly defeat to Bayern Munich:
"I think we are definitely in the market in general. Of course, when players get a long-term injury we need to look into that, but generally we are in the market yes.
"Nobody feels close or far away. I'm not saying anything before there's anything that is completely 100%."
Meanwhile, Frank also assessed Joao Palhinha's debut against his old club.
He added, "I think it's natural. There's been a very short turnaround. He just came back and he hasn't played much in a long time.
"So we got 45 good minutes into him and that's a positive. So he needs a little building up."