Oviedo was on the comeback trail following a hamstring problem and had been due to return in Thursday’s U21 clash with Liverpool.
However, the 25-year-old will now miss Everton’s remaining four Premier League matches and will also be unavailable for Costa Rica at the Gold Cup this June.
“We had a really disappointing situation with Bryan Oviedo in training,” said Everton manager Roberto Martinez. “He has fractured his fifth metatarsal, which is incredible bad luck
“He was in a great moment of form and looking forward to getting back into the squad and finishing the season playing.
“It was a very unfortunate action on his own and he is going to be out for the rest of the season. He was trying to get on the ball and his ankle rolled in a very awkward position.”
Oviedo missed last summer’s World Cup due to a double leg fracture suffered in January 2014.
“It’s not the leg that he had his breakage – it’s his other leg – so it is a completely different injury,” added Martinez. “It’s very unfortunate.
“He’s going to be out for the rest of the season and will miss being with Costa Rica at the Gold Cup.”
Ovideo had worked hard to regain full fitness after breaking his leg in an FA Cup fourth round tie at Stevenage, but despite making his comeback in September, his appearances were sporadic until he got a run of games from the turn of the year before pulling a hamstring in a Europa League tie against Young Boys.
“He has given us a lesson in terms of the mental strength he has,” added Martinez.
“We were really gutted for him but he is the one leading the rehab and he is ready to count the days until he can come back.
“I would say mentally he is in a great place and is a great example to any footballer. He is just desperate to get back and be fully fit.
“It has been such an unfortunate period for him since the game against Stevenage – I am sure this is going to be the last unlucky chapter.”
Everton face Aston Villa at the weekend and are likely to be also missing Steven Pienaar.
“From the squad at the weekend against Manchester United, everyone is fit,” said Martinez. “I don’t think Steven Pienaar will be available.”