The Nationals don't strike out a whole lot vs RHP, but Fernandez's home/road splits are extreme. Fernandez has faced 1,030 batters at home in his career and has a 1.53 ERA, .184/.247/.262 opposing slash line, 2.04 FIP and 32.5% strikeout rate. It's significantly better than his career numbers on the road, which include a 4.07 ERA and 3.05 FIP. Fernandez has pitched much the same this year at home with a 1.77 ERA and 1.73 FIP vs a 4.52 ERA and 3.32 FIP on the road. Fernandez should be a good bet to strike batters out and prevent runs, but being opposed by Tanner Roark will probably make run support iffy. Daniel Murphy may be out of the lineup today as he's missed the past few games with lower body soreness and the Nationals have a huge cushion in the NL East.
Lester faces a Reds team that is starting a reliever turned starter who has a 4.97 ERA and 5.47 FIP. The Reds back him up with a historically bad bullpen. Lester should be in position to get a good amount of run support. He should also prevent runs and get some strikeouts; the Reds offense has been bottom 5 vs lefties by wRC+ and has the 8th highest K%, and Lester's handedness helps reduce their best hitter (Joey Votto)'s effectiveness. Votto has a solid but unspectacular 123 wRC+ vs LHP this year with a below average ISO of .126. Lester has a 2.40 ERA, 3.45 FIP and 25% strikeout rate and is probably the safer play of the two.
For a more risky salary relief play, Archie Bradley ($6,400) gets a huge park upgrade in San Diego to face an awful Padres offense that ranks as the 2nd worst offense by OPS since the ASB and has the highest K% at 26.1%. This combines with the worst full season OPS vs RHP and 2nd highest K%. Bradley has a 3.96 ERA and 3.89 FIP this year away from hitter friendly Chase Field and has about an average K% at 21%. Bradley is risky but can conceivably prevent runs and generate some strikeouts against a bad offense while allowing a fantasy owner to load up on hitters.