With Anthony Rendon doubtful for opening day, who's on third?

VIERA, Fla. - The opening day countdown is on for the Nationals and third baseman Anthony Rendon.

The star injured his knee on March 9 while making a typical Rendon-esque play, diving to his left to stop a sure base bit and then jumping to his feet to fire to Ryan Zimmerman at first base for the out. Rendon actually came up to bat in the bottom half of the sixth inning against the Braves that day and then hasn't played since.

The initial report was just a bruised knee. That was eventually downgraded after a MRI revealed he had a mild sprain of his left medial collateral ligament. We've since found out that after persistent soreness and no real improvement, the team and Rendon opted for a second MRI on Tuesday. According to Nationals manager Matt Williams, no further damage has occurred. But Williams admitted the obvious yesterday: that Rendon's availability for the start of the season in now in jeopardy.

So where do Williams and the Nationals turn to in order to replace the man who finished fifth in the National League MVP vote last season after posting a .287 average with 21 home runs, 84 RBIs and a league-leading 111 runs scored?

Right now, the options include non-roster invitee Ian Stewart, veteran utility man Kevin Frandsen and middle infielder Danny Espinosa.

"Given (Rendon's) absence so far this spring, Ian Stewart's played a lot over there," said Williams. "Danny's playing over there. Franny has played over there. Cutter Dykstra has been over there a little bit, but he's more of a middle guy for us this year. So all of those guys have played for us and played well. They're getting their work in and getting prepared. That's a decision that, if the time comes, it's a decision we'll have to make on who would fill that spot in Anthony's spot, if that happens."

Without question, non-roster invitee Stewart has performed the best this spring. The 29-year-old is hitting .323 with three home runs, one double, and four RBIs in 15 games. His last homer was an absolute bomb off the Tigers' Justin Verlander on Sunday. Stewart owns a career .229 batting average with 61 home runs and 211 RBIs in 511 games across seven seasons in the majors.

"He's a big league player," said Williams. "He understands it. We've seen the stroke come along. The more at-bats he gets, the power starts to come. He's got big power to his pull side. What I have seen that's been most pleasing to me is the couple of hits against the lefties, taking the ball back through the middle or the other way, which tells me he's seeing the ball well and feels good about his timing."

Stewart's best season in the big leagues came with the Rockies in 2009, when he launched 25 home runs and drove in 70 runs.

"It just seems to be working," said Stewart after yesterday's game. "Just hard work paying off, to be honest. It wasn't one thing that I really tried to do over the winter, tried to fix. I guess the only thing I can say is just getting in the cage with (Nats hitting coach Rick) Schu. He got my hands in position to fire, and the rest was up to me at that point."

Stewart was considered a long shot to make the Nats' opening day roster when he showed up at spring training. He hasn't had a productive season in the majors since 2010 and hit just .176 in 24 games with the Angels last season. So even he is a bit surprised at the position he's in with just 11 days remaining before the season opener at Nationals Park on April 6.

"I thought it might be different circumstances, just because you have such a good player in Rendon that was already here, so I thought I'd be battling for more of a bench job, left-handed bat off the bench," said Stewart. "With him going down, I guess there's been an opportunity for more playing time. I don't know how long he's going to be out. It's nice to be able to get the at-bats here and to be able to play and at the same time show them if Anthony's not ready, I can step in."

Williams also likes that Stewart has plenty of big league experience playing second base in addition to third.

Espinosa is very familiar with playing second base in the majors, as well as shortstop. But late last week, Williams sent him in the game to play a couple of innings at third base. It was the first time Espinosa had played the position in a game in his life, although he had spent some time practicing there recently. Williams has since started Espinosa in two games at third as he prepares for scenarios without Rendon.

"He's still learning it," said Williams. "But he's a fantastic athlete. It's the same side of the diamond that he's used to playing at short. The question is getting him game reps, and the spins of the ball and the different angles and things over there that he's learning as we go. We've done a lot of work with him. A lot of extra stuff is going on. That's certainly an option for us."

There's no question of the value Espinosa provides defensively - he's a wizard in the field with what Williams has described as "golden hands". But at the plate, Espinosa has been going through a transformation this spring to become an exclusively right-handed hitter. He's been a switch-hitter his whole life, including his five seasons in the majors. But with little success from the left side of the plate, Williams asked the 27-year-old to focus on digging in from the right side only this spring. It's still very much a work in progress for Espinosa, who is hitting .156 (5-for-32) with one homer, two doubles and two RBIs during the exhibition season.

As we enter crunch time for establishing the roster, Espinosa could be a bubble guy with the team choosing to send him to Triple-A Syracuse to maintain consistent at-bats as he attempts to continue to gain comfort with his new hitting approach. Or maybe his defensive reliability is simply too valuable to not have around especially in late inning situations.

Frandsen, 32, has struggled this spring, managing just three hits in 28 at-bats (.107). He provided great versatility for the Nats off the bench last season. The eight-year veteran played multiple games at left field, first base and second base, in addition to making 12 starts at third. He batted .259 with one homer and 17 RBIs in 105 games. He's signed to one-year, $1 million contract in 2015.

"My role is to play everywhere and to pinch-hit late in games," said Frandsen. "I know that. If they call upon me to start a few games here and there, I'm prepared with all the work I've done and continue to do. The results don't matter for me in spring training as much as being consistent with your barrel and hitting the ball good. I feel like the amount of at-bats all of us have gotten has been great as far as for being prepared for a situation like this with Tony."