Uruguayan Ramirez and USA international Guzan were only able to feature in Middlesbrough's pre-season games outside of the UK, but have now both been declared eligible to play in the country.
"The relevant permissions have now been secured for summer signings Brad Guzan and Gaston Ramirez to play in the UK.
"The duo have only featured on foreign soil for Boro this summer, but the club has now received the green light for the pairing," a club statement read.
Ramirez will start his second spell with Boro after spending the second half of last season on loan at the Riverside, while Brad Guzan has recently signed from Aston Villa.