They are the two changes likely to the side thrashed by 231 runs in the second Test at Port Elizabeth as South Africa levelled the three-match series at 1-1.
Lehmann will wait to see the wicket at Cape Town before deciding whether all-rounder Watson returns.
Bowling on the slower and less bouncy St George's Park wicket, Mitchell Johnson was unable to have the same impact as he did in the first Test, during which he took 12 wickets.
Lehmann said having an additional bowling option would be useful, but Watson needed to prove his fitness.
"As long as he's (Watson) bowling overs and he's fit, we would love to have that extra bowler," he said on Monday.
"We'll have to wait and see how that comes along. He seems all right, batting doesn't seem an issue. Hopefully he'll be able to bowl.
"At the end of the day, if we have to fit Shane Watson in and someone misses out, it will be really unlucky.
"It depends on whether we need the fifth bowler or we don't, and we'll have to wait until we see the wicket. And he's got to be fully fit. All those things come into it."
One of Shaun Marsh or Alex Doolan, who both failed in the second Test after a bright start to the series, is likely to make way.
Pattinson shapes as being the other inclusion, with Ryan Harris or Peter Siddle perhaps tiring during the second Test.
Siddle has bowled 82 overs for five wickets at 54.6, while Harris (69.4 overs, three wickets at 74.33) has also struggled.
"We'll have a look at it, velocity's a big thing for us," Lehmann said.
"You certainly need pace, we've seen that with Johnson, Harris and Siddle when he's up and running. We'll have a look at that over the next few days. We'll see how they pull up."