Hairston, 29, has hit .247 (20-for-81) with two doubles, six home runs, 14 RBIs and 14 runs scored in 32 games for the Padres this year.
"It's healing and I'm not getting that sharp pain when I walk," said Hairston, who injured his leg Saturday coming in for a fly ball hit by the Dodgers' Blake DeWitt.
"It is something that I have to be really careful with because hamstring injuries are something that sometimes you feel you're OK and then you try and go out and test it too soon and reinjure it again," said Hairston. "I'm going to be smart about it so I can take care of it.
"There is plenty of time left in the season; you don't need to really rush to jeopardize a couple months. This is just going to be a bump in the road.
"I feel they made the right move [in calling up a player]. With a hamstring injury it usually takes a full two weeks to recover from that."