The Scottish champions were drawn against Barca in the last 16 of the competition - the first match of which is at Celtic Park on February 20, with the return game a fortnight later in Spain. Rangers took more than 20,000 fans to Spain when they faced the Catalans in the group stages of the competition last month and McDonald believes a good result at Parkhead will have the Celtic fans reaching for their passports.
He said: "If we were to get a great result, I couldn´t begin to imagine how many Celtic fans would make the journey over to Barcelona. There would be an invasion, much like when Rangers went to Barcelona - maybe bigger."
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