Panesar was released by his county, Sussex, earlier this week after he was fined by police for his part in a drunken incident in Brighton that involved the player urinating over nightclub bouncers in the early hours of the morning.
But while Cook admitted that Panesar had "let the England shirt down", he also acknowledged his qualities as a bowler and let him know that he will be welcomed back into the side if he can demonstrate he has made the necessary "changes in his life".
"What's happened over the last couple of weeks has been disappointing," Cook said. "I've spoken to him - he actually rang me - which shows he knows he's done wrong and let the England shirt down a bit.
"But he recognises he needs a bit of a change in his life which I think is always the first step.
"The bottom line is that we need Monty back bowling as well as he can bowl. The way back is through taking wickets and he knows that. We know he has that pedigree in international cricket."
Panesar has started the process of change by joining Cook's county side, Essex. And while Cook admitted that move had taken him by surprise, he welcomed it.
"I didn't know he was going to Essex," Cook said. "I thought he was going to Northants. I'm not the be-all and end-all at Essex. But I'm glad we get a quality spinner and I'm going to get a nice good look at him at Essex."
Apart from taking wickets, though, Panesar will also need to demonstrate that he is committed to team success and has no problems with alcohol. Panesar had been dropped by Sussex earlier in the season for a poor on-field attitude.
"I don't really know all the details," Cook said. "But he has that side of his life that he definitely needs to get right because we know what an off-field life can do to you.
"Everyone is saying, 'Is the door shut?' It's certainly not, but he knows what he must do to get back.
"It was not the most pleasant phone call for him to make - to the England captain to apologise. But we know the class Monty has as a bowler and what he can do when he gets it right. We've seen that time and again. He just has to go back to basics, work as hard as he can and do what he does best - which is taking wickets. If he does it for Essex, it's even better for me."