"It was a big shock when I got the call, but I'm over the moon," Cowie told the Scottish Sun.
"It's been a bit of a whirlwind. I got a call from my manager to tell me the news and now I can't wait to go away with the squad.
"I know there have been a lot of withdrawals, but that doesn't take anything away."
Full squad for the match that kicks off on Saturday morning UK time.Goalkeepers: Craig Gordon (Sunderland), Jamie Langfield (Aberdeen), David Marshall (Cardiff City)
Defenders: Darren Barr (Falkirk), Christophe Berra (Wolverhampton Wanderers), Gary Caldwell (Celtic), Lee Wallace (Hearts), Stephen McManus (Celtic), Steven Whittaker (Rangers)
Midfielders: Charles Adam (Blackpool), Graham Dorrans (West Bromwich Albion), Craig Conway (Dundee United), Ross Wallace (Preston North End), Stephen Hughes (Norwich City), Don Cowie (Watford)
Forwards: Steven Fletcher (Burnley), Lee Miller (Aberdeen), Derek Riordan (Hibernian)