Shane Long took his goalscoring tally for the Royals to 51 after he scored a first-half double to see off Pompey who had Ricardo Rocha sent off before the break.
The result moved Reading into sixth on goal difference from Nottingham Forest but McDermott remained realistic.
He said: "We knew three points would get us into the top six and we have kept a clean sheet so I am happy with that.
"In the second half we didn't play as well as we might and we didn't see the game out as well as we would like."
Source: PA
Source: PA