The 22-year-old, who joined Fulham's academy in 2009, has played four times for the Cottagers and spent a month on loan at Accrington last season.
The Finn also had spells on loan with Maidenhead United, Cambridge United and Finnish Premier League side FC Lahti.
The 6ft 5in goalkeeper has represented Finland at several youth levels and made his senior debut in 2013.
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Riaz described England's middle order as "fragile" before the first Test at Lord's starting on Thursday.
"It's the same before every series - you generally get a bit of trash talk," said Root, who will bat at three.
"It's getting a bit like boxing now. We will have to wait and see how things develop and see if the plans work."
Boxing is notorious for fighters' pre-match verbal contests aimed at intimidating opponents.
James Vince, who has played only three Tests, and the recalled Gary Ballance will bat at four and five for England when the four-Test series gets under way.
Former England captain Michael Vaughan said Vince should have been promoted to three to enable Root to stay at four.
England vice-captain Root, 25, averages 50.36 in 21 innings at number four and only 28.71 in eight innings at three.
Root, 25, said it is "a great opportunity to bat a bit longer" and that captain Alastair Cook, coach Trevor Bayliss and the selectors gave him time "to get my head around it".
He added: "It's about taking that opportunity for responsibility as a senior member of the side, to lay some good foundations for us to make some big scores."
Nick Compton batted at three for England in the 2-0 series win over Sri Lanka, but announced he was taking a break from cricket after scoring only 51 runs in five innings.
Vince managed 54 runs batting at five against Sri Lanka, his debut Test series.
Ballance has not played for England since being dropped last July, but made 132 for Yorkshire against Middlesex this month in his most recent first-class game.
Root said Vince and Ballance had a "great opportunity" to prove Riaz wrong, adding: "Gary's in fine form - he's just got a hundred for Yorkshire.
"Vincey's a brilliant player. He made his first innings for England in the one-day series against Sri Lanka - he made a brilliant fifty. So they'll take confidence."
Pakistan pace bowler Mohammad Amir will play his first Test since serving a five-year ban and three-month prison sentence for spot-fixing at Lord's in 2010.
England captain Alastair Cook has warned the 24-year-old, who took four wickets in Pakistan's opening tour game against Somerset, to expect a "reaction" from the crowd this week.
But Riaz said: "Mentally he's very strong, and he's very eager to perform.
"The place where it all began, he will perform at that place and make sure his name is reborn."
Stevenage have signed Finland goalkeeper Jesse Joronen on a six-month loan from Championship side Fulham.
[NEXT_CONCEPT]
England batsman Joe Root said cricket is becoming like boxing as he dismissed Pakistan bowler Wahab Riaz's "trash talk".