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Speaking ahead of the launch of Finding Jack Charlton,a film about the former England World Cup winner and Republic of Ireland manager and his struggles with dementia, Townsend hailed the impact of his old club's current captain.
"I think it's starting to get to the point where rather than a toss of the coin for Gareth Southgate where it's shall I, shan't I, but he'll have to play Grealish," Townsend told Goal.
"The way that he can carry the ball, the way that he can travel across the ground, in international football you can't just attack and defend. England are a bit like that.
"They could do with someone to just hang on to it and link the team, to allow the team to retain possession of the ball and wait for the right moment and do something in the final third.
"They have such great energy and pace and skill in the front line, but they don't have anyone quite like Grealish.
"They could do with someone like him. They would have better spells in games against better teams with him because he does keep the ball very well."
During the international break, Grealish played for England against the Republic of Ireland, having representedthe Boys in Green from Under-17 to U21 level.
West Ham's Declan Rice was left on the bench for that friendly, but started both of England's Nations League games during the break, despite having featured in three friendlies for Ireland back in 2018.
Townsend believes that Ireland missed out on two great players when the pairdecided to play for England at senior level, saying that Rice should never have been allowed to switch from Ireland and, as an English-born Irishman, he never would have made the same decision.
"Once you commit to a country and play an international friendly, once you play any international fixture, that should preclude you from deciding that 'Actually, I don't want to play for this team now, I want to go and do something else,'" Townsend said.
"I don't agree with that rule at all. Had that rule not existed, I think both Declan Rice and Jack Grealish would both have been perfectly happy pulling on the green jersey and getting on with it and doing everything they can to help us qualify for a major tournament.
"They're both very significant players for their clubs. Declan is already well admired and well respected by Gareth Southgate and I don't think it will be long before Jack Grealish is in a similar situation. England can't avoid him for much longer; he's too good.
"Those two boys would have made a lot of difference not only to (Ireland manager) Stephen Kenny's life, but to Mick McCarthy's and maybe even the managers before that.
"It's the rules and they're not breaking any rules. They haven't stepped out of line. They've done what they wanted to do. They've made a decision and they have to live with it.
"It's not a decision I would have made. I would never have done that even if I was allowed to do that. Never, never."
Leeds striker Patrick Bamford is another English-born player eligible to play for Ireland, but has so far resisted the advances of various managers to play in green.
Townsend feels that there is no point in current Ireland boss Kenny chasing him to play for the country, especially if the uncapped 27-year-old'sheart is not in it.
"You've got to have players who want to play for you. You've got to have a want to do it," Townsend continued.
"If Patrick is not comfortable with representing Ireland, then he has no need to do it.
"Any player, particularly one like him who is enjoying a good period at the moment, would be a very useful addition to our numbers and our squad.
"The guys have got to do it. But if they don't want to pull the shirt on or wear green, then don't chase them."