I'm not very familiar with Bertans but he doesn't seem like he's able to the bigger and more athletic small forwards in the NBA. I guess thats why he's been compared to Daye (NBA Draft Net) a bit of a tweener. Not a perfect fit at either position.
I agree, Splitter doesn't seem built to play 30+ mpg over the course of a 82 game season + postseason. Valid point as well about Milsap and Diaw (also, Jean Charles) all not being able to play center. Unfortunately their aren't very many realtistic options on the FA market.
There aren't many bigs that can play both PF and C on the open market, and anywhere in the Spurs range. Kevin Garnett can fit that role but there are plenty of reasons why that wouldn't work out; he will be 39 by next offseason & entering his 20th season , likely needs to take the vet. min, Duncan doesn't like Garnett (SI mentioned this in an article a couple of years ago) and Garnett hasn't come off the bench since Bill Clinton was president ('95-'96 season more than 1300 starts since then)
Jordan Hill can play some of both and shot the ball well from outside of the paint last season. He is very productive in limited minutes. He has a team option for next season so the Lakers could decline his deal in order to become players next off season.
In a center, The Spurs need a big who can defend & board, pass has a solid BBIQ and an ablity to finish around the rim.
Another option could be finding a 3rd center (assuming Baynes is back for cheap) in the 2015 NBA draft, which is supposed to be very big man heavy.
(projected to be drafted in the first round)
Bleacher Report-14 PF/Cs
NBA Draft Net-13 PF/Cs
Draft Express-12 PF/Cs
I like Milsap's fit, in part because of floor spacing ability
he hit 40.6% of his 3 FGA per game from 10-22 feet (above average)
and he stretched his jumper out to 3 point range, 35.8% 3pt of his 3 FGA per game
Horford has had some health issues but is otherwise a very good player. I don't think both Tim and Manu will return after next season but I could see Manu returning for another year. Its hard to imagine the Spurs being able to have enough cap space to outbid another team in two seasons, while still being a playoff team.