Dowson started England's first two matches during the Six Nations, against Scotland and Italy, but lost his place to Ben Morgan, who went on to establish himself as one of the stars of the competition.
Morgan is currently recovering from a hamstring injury so Dowson was sure to start in the non-cap international at Twickenham, and he has been selected as captain ahead of Saints teammate Dylan Hartley.
And Rowntree believes the former England Saxons captain is one of the senior figures in the dressing room.
"Phil is one of the key members of our leadership group," said Rowntree.
"He coped with the disappointment [of losing his place in the team] very well.
"There are a few guys we could give the captaincy to but we feel Dows is right for this game. This is a proper game, as intense and as big as a Six Nations game for us."