The former England one-day batsman is ineligible to play in the clash, under terms of his loan move to Gloucestershire last month.
Mustard – in his benefit year at Durham – has been a key member of the county’s one day success in recent seasons.
But having slipped out of the reckoning at the Riverside, he gladly accepted the offer to get more game time under his belt down south – only to be thwarted when the quarter-final draw was made.
Mustard said: “When the draw came out, myself and Richard Dawson looked at each other and had a bit of a giggle. That’s all we could do as it was in the loan agreement that I couldn’t play if it happened, so we’ve just taken it on the chin.”
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While he still has a foot in both camps, the Sunderland-born wicketkeeper believes home advantage could be key to Gloucestershire’s bid to reach Finals Day.
He added: “Durham’s away form has been the same in all the years I’ve played there, and they do travel the most miles.
“T20 is often about one individual performing really well, and both teams have people who can do that so it’ll be about which team has the most people doing the most damage.
“It’s a huge carrot for both teams, in T20 neither team has ever won it. It looks to be an evenly matched game.”
Durham go in search of their second appearance at Finals Day, the first coming in 2008. The game is scheduled to start at 6.30pm and will be televised live on Sky Sports.