Two players tested positive for the virus and another two have been ruled as close contacts ahead of Sunday's Hampden clash with St Mirren.
McCart said: "It is a shock and it is really disappointing for the boys that it's happened to.
"We are gutted. The full season we have been really lucky with it, thankfully nobody has really caught it.
"We are just coming to a really pivotal moment in the season and unfortunately it has arisen.
"But it gives us maybe even a little bit more motivation to go and win the game and they will be back for the final.
"As the gaffer has said to us, you have just got to put that out of your mind right now and focus on the game because it's an extremely tough game against St Mirren and it's such a big opportunity, it would be a shame to let it slip. So we just have to focus."
A statement from Scott Boyd, head of football operations, earlier confirmed the blow.
Boyd said: "The football club can today confirm that two members of our playing staff have tested positive for Covid-19. They will both be missing from the Scottish Cup semi-final on Sunday against St Mirren.
"Thankfully, after the latest round of testing, all results were negative.
"However, as a result of the positive tests, two other players have been forced to self-isolate due to Government guidelines. The two players in question tested negative.
"We feel our staff adhered to the guidelines and this has been largely outwith our control.
"It's very unfortunate for four of our players to miss out on the important game on Sunday.
"However, we wish all of them well and their health and wellbeing will always be our priority."
Manager Callum Davidson will take some comfort in knowing that his side have been largely unaffected by making sweeping changes in recent months.
Saints made seven changes following their quarter-final win over Rangers but still beat Hibernian at Easter Road last weekend.
"We all know the style we want to play," McCart said. "Any slight tweak in formation or personnel, doesn't really affect anyone.
"Whoever comes in fulfils the duties as well as anyone so that's really positive for us."