The Scots have been struggling, as they have not won any of their last nine competitive games. Their recent defeat against Croatia in Zagreb was particularly disappointing, as a potential late equaliser was disallowed for offside after a VAR review.
They return to Hampden Park on Tuesday to take on Portugal as they look for their first victory in Group A1.
Cristiano Ronaldo netted the late winner when the sides met in Lisbon in September, while Scotland had already lost to Poland by a one-goal margin in their group opener in Glasgow.
Rangers midfielder Connor Barron is the latest call-up to the squad with Celtic's James Forrest withdrawing injured.
TrendingDespite the disappointing start and a number of regulars unavailable, Clarke insists his players are confident of taking something against Roberto Martinez's side.
"They do believe in themselves," Clarke said.
"The players understand we're playing difficult opponents, they understand the squad could be stronger.
"I don't speak about it too much because I think it's disrespectful to the boys that are in the squad. We have to concentrate on who we have here.
"We have to go out against Portugal and be good at everything in the game, to go out and get the result that might just change the mood about the place."
He added: "After 20 years in the international wilderness, this group of players have been to two major tournaments.
"They've got promotion to the top level of this Nations League. I don't understand why people would be doubting this group.
"It's a tough moment, no doubt about it. But we believe in ourselves."
Goalkeepers: Craig Gordon (Hearts), Jon McCracken (Dundee), Robby McCrorie (Kilmarnock)
Defenders: Nicky Devlin (Aberdeen), Grant Hanley (Norwich), Liam Lindsay (Preston North End), Ryan Porteous (Watford), Anthony Ralston (Celtic), Andrew Robertson (Liverpool), John Souttar (Rangers), Jack McKenzie (Aberdeen)
Midfielders: Ryan Christie (Bournemouth), Ben Doak (Middlesbrough), Ryan Gauld (Vancouver Whitecaps), Billy Gilmour (Napoli), Andy Irving (West Ham), Kenny McLean (Norwich), Scott McTominay (Napoli), Lewis Morgan (New York Red Bulls), Connor Barron (Rangers)
Forwards: Ché Adams (Torino), Lyndon Dykes (Birmingham City), Kevin Nisbet (Aberdeen)
Stream live with no contract on a Month or Day membership on NOW. Instant access to live action from the Scottish Premiership, Premier League, EFL, F1 and so much more.
Find out more here...