Reliever Ryan Mattheus will begin throwing in the bullpen again Sunday, with the intention to return from the disabled list by Sept. 20. If all goes well, Mattheus will throw one bullpen session every other day for a week starting Sunday, then throw a simulated game, then return to the bullpen.

The Nationals placed Mattheus on the disabled on Aug. 26 after he experienced stiffness in his right shoulder. Mattheus felt no pain, but he threw with little velocity on his fastball. Mattheus, a rookie, has made 54 appearances in his first full season back from Tommy John surgery.

When the Nationals placed him on the disabled list, Mattheus said he hoped to return this year in order to build confidence in his health heading into spring training 2012. In 32 major league appearances, Mattheus compiled a 2.40 ERA in 30 innings, striking out 11 and walking 14.