Wilkinson, who is 30 on the day the 2009 Lions are due to head to South Africa, dislocated his left knee-cap against Gloucester in October and has been recovering from an operation since then.
"Jonny has been training hard, which he needed to do, but there has been a slight reaction to that which looks like keeping him out of contention for this weekend," said Bates.
"To involve him on Saturday would be hasty, but he's very close, and it might well be that he gets a run next Friday at home to Leicester.
"He has had a long lay-off after quite a significant operation. We just need to go through the whole process of testing it against everything he might encounter in a game to make sure that he is 100 per cent.
"This week we have upped the intensity a little, and the knee has just swollen up a bit as a result. I think that is absolutely to be expected, it's just part of the process of putting the knee through its paces.
"We have to accept that it's not going to be automatically right every time. He is making fantastic progress, but we just need to make sure he's ready before we put him back on the field.
"I think Jonny recognises that the team is going very well right now, and it's not necessarily going to be a case of saying you're fit now, so go straight in to the starting XV.
"There's been no firm decision made, but it would be inconsistent as a coach to say here's a guy in Tom May who has been playing really well at No10, and now let's just restructure everything to accommodate one player returning.
"I think, on balance, we need to reintroduce Jonny safely. I don't think we're in a position to say go and start for the team straightaway.
"I would guess we will introduce him from the bench when the time comes, and see how we go from there."
One man who will be watching Wilkinson's progress very closely is Lions head coach Ian McGeechan. With no outstanding candidates for the outside half berth for the Test series against the world champion Springboks it could be that Wilkinson makes a third tour.