WASHINGTON -- Nationals right-hander Stephen Strasburg is expected to throw a bullpen session on Friday, and if things go according to plan, he will start his rehab assignment on Sunday for Class A Hagerstown, according to manager Davey Johnson.

The news comes a day after Strasburg threw a simulated game in Viera, Fla.

Strasburg is expected to pitch one or two innings for Hagerstown and make at least four rehab starts before making his season debut with the Nationals sometime in September. Strasburg could replace right-hander Jordan Zimmermann in the big league rotation; Zimmermann has a limit of 160 innings and is already at 132 2/3 innings.

Strasburg, who is recovering from Tommy John surgery, is considered the future ace of Washington's pitching staff. In his short stint with the Nationals last year, the No. 1 overall pick in the 2009 First-Year Player Draft was 5-3 with a 2.91 ERA in 12 starts. He struck out 92 batters in 68 innings.

"Having Stephen back ... is huge. It's like getting a free-agent pitcher," third baseman Ryan Zimmerman said. "He has worked really hard. He is in incredible shape. He really wants to come back and better than he was before. Obviously, he is very talented, but he works his butt off, and we are lucky to get a good person."

Pudge steaming toward rehab assignment

WASHINGTON -- Nationals catcher Ivan Rodriguez, on the disabled list since July 7 because of a strained right oblique, will start a rehab assignment this weekend, most likely for Class A Hagerstown. If that's the case, he could be catching right-hander Stephen Strasburg, who will make his first rehab assignment Sunday.

If things go according to plan, Rodriguez could be activated from the disabled list during the next homestand, which starts on Aug. 16 against the Reds.

As the backup catcher, Rodriguez has played in 39 games, hitting .214 with two home runs and 19 RBIs.