Doug Fister could start playing catch Thursday. (Photo by John McDonnell/The Washington Post)

Doug Fister has rested his strained right lat muscle for six days and could resume playing catch as soon as Thursday, Nationals Manager Matt Williams said. The right-hander was making a start in a minor league spring training game last Thursday, returning from earlier elbow inflammation, when he exited after one inning because of discomfort in his side.

Fister said he did no activity for five days, but has resumed some cardio exercises  in recent days. So far, Fister said his lat strain is improving.

“It’s feeling better,” he said. “Working, doing some exercises to strengthen it back up.”

Fister was prescribed a five-day rest period from throwing and placed on the 15-day disabled list, backdated to March 23. Because the starter, acquired in the offseason from Detroit, was already behind schedule thanks to his elbow issue, his return could last anywhere from three weeks to a month. It hinges on his arm strength and health.

Although he built up some arm strength prior to the rest, he will have started essentially from the beginning because the Nationals are being cautious. When he first plays catch, he will throw from a distance of 60 feet before progressing to 90 and 120 feet before he throw from a mound again.

“What we don’t want is another setback,” Williams said. “So have to make sure that we do it properly. That he feels good about it, too.”

>>> Right-handed reliever Erik Davis underwent Tommy John surgery on Tuesday in Los Angeles. The procedure was performed by Neal ElAttrache. Davis, who made his major league debut last summer, suffered an elbow strain early in spring training and was placed on the 60-day disabled list on Feb. 13.

The Nationals first tried rehabbing the injury but that didn’t work. After a length shutdown, Davis threw again last week but lasted only one day. He underwent surgery and was told he had “substantial damage” to his right  ulnar collateral ligament (UCL). He will miss this entire season.