The Frenchman has made five appearances for the club and Megson has been impressed by the centre-half's ability on the ball.
However, he is keen to assess how Puygrenier copes with the pace of the Premier League before reaching a decision on his long-term future.
"There is an amount that Zenit would want," said Megson in the Daily Mirror. "We have already paid them a loan fee.
"It's not about match fitness with Seb, it's sharpness."
He added: "On the ball, he's probably as good a defender as we've got here.
"One of the things people talk about when foreign players come over here is the technique they have got. But it is entirely different using that technique when you are going at 100 miles per hour and have someone breathing down your neck for 90 minutes.
"It takes a bit of time and it's only to be expected that Seb will need time too.
"It's one of the issues you get when you bring people over to this country for the first time. It's a lot faster, a lot more physical and you get no time on the ball."