The England back-row forward sustained the injury after slipping in training and was removed from the match-day 23 after a scan this morning.
Lawes place in the starting line-up is taken my Mark Wilson, who had been dropped from the squad following the Scotland defeat, while the only other change to the starting XV from the side who defeated Italy 41-18 two weekends ago is Jamie George instead of Luke Cowan-Dickie.
The only other change comes on the bench with the uncapped George Martin brought in, Jack Willis now facing a lengthy spell on the sidelines after suffering a nasty injury after coming off the bench to score against the Azzurri.
Of the Lawes injury, England head coach Eddie Jones said: “In the first part of training yesterday, he slipped over and felt something in his pec. We weren’t sure of the significance of the injury so he had a scan this morning.
“He’s got a little bit of damage there so that needs further diagnosis which will be done over the next few days.”
Jones said it was unclear at this stage of the severity of the injury. He added: “We’ll have a clearer picture over his participation in the tournament. He’s done some damage to the pec muscle. Sometimes that can be serious, sometimes it’s not so serious. We’ll have to wait and see.”
The enforced change means that England will opt for a different approach from the scrum, Jones describing Lawes’ role as a “jumping and running” one in contrast to a big defensive, clear-out focused game from Wilson.
Some of England’s most established stars have come under criticism for their performances against both Scotland and Italy, most notably captain Owen Farrell and No8 Billy Vunipola, both of whom have had a lack of game time at club level with Championship side Saracens.
But Farrell, who shifted from fly-half against Scotland to outside centre against Italy to make way for George Ford, retains his place as expected for the trip to the Principality Stadium, so too Vunipola.
And despite the lack of crowds for Saturday’s game, Jones said he still expected an edge to the game.
“There always is,” he said. “That’s the allure of the Six Nations. They’re tight contests going down to the last moment. It’s the sort of game you’ve got to not win once but two or three times.”