Morse began his career in Seattle at shortstop, but quickly outgrew the position.
He last played shortstop for two innings in 2007, and most of his starts in Washington have come in right field.
"I'll always think of shortstop as my position," Morse said. "In my heart, I'm a shortstop, but my body tells me I'm not."
Entering Wednesday's game against the Phillies, Morse started 10 consecutive games at first base. With Adam LaRoche on the 15-day disabled list, Morse took over the role as the Nationals' everyday first baseman and has yet to make an error.
"I'm not going out there trying to fill Adam's spot," Morse said. "Adam LaRoche is Adam LaRoche. He's a great player and I can't wait until he comes back because he's a big part of the team. In the meantime I'll do my best."
While LaRoche was known for his glove, he struggled at the plate. Morse just capped a month of May in which he hit .403 with six homers and 15 RBIs in 22 games.
Riggleman said Morse can still play third base, but with the Nationals' depth at the position he is lower on the pecking order and more likely to play first base and outfield.