Townsend and Shelvey joined relegation-threatened Newcastle United in last month’s transfer window.
Shelvey – who set up the winning goal, scored by Aleksandar Mitrovic – was outstanding against West Brom.
And Lascelles feels the England internationals will make a difference on and off the pitch with their “leadership and determination”.
The West Brom result saw Newcastle climb out of the relegation zone ahead of Saturday’s game against champions Chelsea at Stamford Bridge.
“Obviously with the English boys, it’s nice to have them around as well,” said Lascelles, who has trained with the senior England team himself.
“In Andros and Jonjo, we’ve brought in two big characters, which I think we needed.
“I think we’ve lacked a bit of leadership and a bit of determination on the pitch recently.
“So they’ve definitely given us a bit of a lift, and hopefully we can get three points against Chelsea.”
Lascelles captained United’s Under-21s in Monday night’s 2-1 defeat to Stoke City at St James’s Park.
United had defender Jamie Sterry sent off early in their game, and the visitors’ extra man told late in the match, decided by a 77th-minute goal from Lewis Banks.
“Even when we went down to 10 men, I thought we defended really well,” said Lascelles.
“To be fair, we went through a few spells where we played really well, but I think you can’t really defend for that long and expect nothing to happen.”
On wearing the captain’s armband, the 22-year-old added: “It’s always nice. Obviously, I got good fitness out of the game which I think is what I needed, because I didn’t play at the weekend.
“Also, it’s good to play three at the back against 11 men, because for the majority of the game we played with 10 men. I think there’s a learning curve for me and the rest of the boys.”