Martinez is reportedly Barca's prime transfer target as the Spanish giants look to sign a long-term replacement for veteran forward Luis Suarez.
Also linked to Real Madrid and Manchester City, Martinez had scored 16 goals in 31 games across all competitions before the coronavirus pandemic brought sport to a standstill.
But former Madrid, Milan and Juventus head coach Capello urged the Argentina international to remain at Inter.
"If it were me I'd stay at Inter," Capello told La Gazzetta dello Sport.
"At Barcelona he'd be a substitute."
Martinez has a €111million release clause and Inter are unwilling to accept anything less amid Barca's reported interest.
But the COVID-19 crisis, which has wreaked havoc across the world, is set to impact football finances and the transfer market.
"The market will be revolutionised," Capello added. "Absurd numbers had been seen, but now we'll return to a more reasonable level.
"It seems to me that the only team in Spain who won't be destroyed by the pandemic are Real Madrid.
"Barcelona and Atletico Madrid are going to suffer a lot."