Underwood and the Canadians will be facing a dangerous but likely tired Fijian side at Twin Elm Rugby Park in Ottawa.
The Pacific Islanders have had to travel 16 time zones since knocking off 15th-ranked Japan 22-8 on Saturday on a waterlogged field at Lautoka, Fiji.
The win moved Fiji into 13th place in the IRB world rankings, dropping Canada one spot to No. 14.
Canadian coach Kieran Crowley called the date with Fiji "a massive challenge."
That's because the bulk of the Fijian squad plays professionally in Europe. And they showed off a powerful scrum and a speedy set of backs against Japan.
Marquee names include captain Akapusi Qera, who plays for England's Gloucester, and former sevens skipper Sireli Bobo (Racing Metro, France), who needed just 33 seconds to score in his return to the full side.
The Fijian side against Japan was an experienced one, with an average age of 29.
The Canadians, who opened up their tournament with a 16-9 win over the 16th-ranked Americans in Edmonton on Saturday, face a hectic schedule of their own.
After Fiji, they host No. 11 Tonga on Saturday in Kingston, Ont., with games to follow against No. 9 Ireland on June 15 in Toronto and Japan on June 19 in Nagoya.
All but the Ireland game are part of the newly expanded Pacific Nations Cup.
It's five games in 25 days, prompting Crowley to manage his squad carefully. The former All Black has introduced Underwood and six others into his starting 15 against Fiji.
None of the changes are injury-enforced. Crowley is looking to use his full squad to better manage the crowded schedule and to blood some of his younger players.
"We've got to find out about them," he said.
That includes Underwood, who turned 22 Monday. He plays for Queen's University and the Ontario Blues and has represented Canada twice at the under-20 IRB Junior World Trophy.
"He's worked hard in training and he's certainly a player's who got a bit of ability, I think," said Crowley. "It's going to be interesting to see how he goes."
"A silky runner," said Mark Winokur, general manager of the Ontario Blues.
The average age of the Canadian side that faced the U.S. was 25.
More veterans are joining the team as the tournament progresses.
London Irish forward Jebb Sinclair and sevens star Nathan Hirayama will be on the bench Wednesday after missing the U.S. game.
Veteran fullback James Pritchard is also back in camp after recovering from a broken arm and finishing out his club duties with the Bedford Blues in England.
Canada is working towards a crucial home-and-away World Cup qualifying series in August against the U.S.
Andrew Tiedemann, Castaway Wanderers, St. Albert, Alta.; Ryan Hamilton, Capilano RFC, Vancouver; Jason Marshall, Atlantique Stade Rochelais (France), Vancouver; Jon Phelan, Ste-Anne-de-Bellevue, Montreal; Tyler Hotson, London Scottish (England), Vancouver; Tyler Ardron, James Bay AA; Lakefield, Ont.; Nanyak Dala, Castaway Wanderers, Saskatoon; Aaron Carpenter (capt.), Cornish Pirates (England), Brantford, Ont.; Phil Mack, UVIC Vikes, Victoria; Liam Underwood, Queens University, Toronto; Taylor Paris, SU Agen Lot-et-Garonne (France), Barrie, Ont.; Harry Jones, Capilano RFC, Vancouver; Ciaran Hearn, Castaway Wanderers, Conception Bay, N.L.; Matt Evans, Cornish Pirates (England), Maple Bay, B.C.; Connor Braid, Doncaster Knights (England), Victoria.
Ray Barkwill, Niagara Wasps, Niagara Falls, Ont.; Hubert Buydens, Saskatoon Wild Oats, Saskatoon; Tom Dolezel, London St. George's, London, Ont.; Jebb Sinclair, London Irish (England), Fredericton; John Moonlight, James Bay AA; Pickering, Ont.; Sean White, James Bay AA, Victoria; Nathan Hirayama, UVIC Vikes, Richmond, B.C.; Nick Blevins, Calgary Hornets, Calgary.