Ally McCoist's side head into their last-eight clash with Albion Rovers strongly fancied to continue their fine form and set up an Ibrox fixture that would put them just 90 minutes away from the end-of-season finale.
There is little to choose in the betting between the Light Blues and Foster's former club Aberdeen, who are currently second in the Premiership standings and also in the League Cup final.
But the defender reckons the Gers should not be high up in the market with the likes of Dundee United, St Johnstone and Inverness Caley Thistle still left in the competition.
Foster said: "I don't think you can call us favourites if I am honest. There are three or four SPFL (Premiership) teams left in, and I don't think it would be fair on them for us to be called favourites.
"We have done well to get to the quarter-finals and we will have another tough game tomorrow.
"If we get into the semi-finals we are at least 4/1. But I don't think we are favourites."
Rangers may be playing down their chances of going all the way and lifting the silverware at Parkhead in May, but that won't stop the Light Blue legions dreaming of what would be a remarkable cup success.
McCoist has never taken his side to Scottish football's pinnacle competition and Foster is confident the League One champions-in-waiting could take on the best teams remaining as they bid for glory this term.
He said: "The Scottish Cup final is the biggest showpiece game in this country and everyone wants to play in it.
"We're two games away from that stage and that, in itself, is a massive achievement for us. If we get to the final it would be incredible for everyone connected with the club.
"Obviously, you don't want to get too far ahead of yourself, but I have thought about the semi-final - and I don't know whether I'd rather play Aberdeen then or in the final itself.
"Let's just beat Albion Rovers first and then we can see what happens.
"You want to play against the best teams you can because that will give you an idea about how good you are.
"We're a confident group of lads and we know we have a strong squad so it would be nice to face one of the teams who are up there in the Premiership. It would be a good gauge for us."