Some have floated the idea of Newcastle offering Isak an improved contract with a release clause, keeping him for another season. Pennant, however, dismissed the plan as risky, pointing to the unpredictability of football careers.
"There's no guarantee, he could get injured. There's no guarantee Liverpool will want to go back in next year," he said.
"We've seen what happened to James Maddison. It's very difficult to see. He's out for the season now. All it takes is one injury to then your career, literally go, get railroaded off track. So it's all good saying we'll give him this in the next season, but it's all ifs and buts we don't know what's around the corner."
According to Pennant, cashing in now would secure a massive financial return while avoiding the possibility of diminishing performances from a frustrated player.
"I think it will be in everyone's best interest if Newcastle took £110m," he said. "Well, he's unhappy, so [if he stays] you might not be getting your best player."