Rojo had a though beggining on Sporting’s first team. He spent half of his first season playing really, really bad. His positioning was really bad, and as a team we were a total chaos. I mean, anyone could see he was technicly good (considering he plays as a central defender) and his phisical skills were really good (he was fast and really strong, which is a very interesting mix).
Then, we got Jesualdo Ferreira as coach. He is not in his best moment, and he is a bit far from the top coaches of our League, but he could manage to bring some organization to our team. We started to play as a team, which we didn’t do before. That was important for Rojo. I mean, he clearly needed some orientation, because he was totally lost in the field when he played. Rojo’s partner became Tiago Ilori (which signed Liverpool in the end of the season), a former Sporting player from our Academy. Together, they made a really good partnership. Ilori had way more class than Rojo (Liverpool was right signing him). But the true is Rojo improved a lot. He finally managed to put his natural skills (strenght, speed, technique) at the service of the team. He was no more lost: finally, we had a coach that could organize our defense.
Last summer, we made changes in our Club again, and there came a new coach: Leonardo Jardim. He is, currently, Monaco’s coach. And he is one of the top coaches in the World working on the team’s defensive process. That’s what he does best, actually. And Rojo had finally a full year playing really well. I mean, he made mistakes last season. It’s obvious. Any player (or, at least, most of the players) does. But he played good. His head was in the right spot. In last season’s second half he started to improve, and this year it continued to happen. With so many competitive games for him (Portuguese League has some really though teams - considering it’s not a top3 or even top5 League), he improved phisicly. Right now, it’s really hard for any forward to win a 1x1 situation against Rojo: he has a good jumping, he his really strong when he has to make contact with the forward and he is really fast. He was also way more confident and way more mature. That means right now he is a really good defender.
Why have I mentioned so many times the coaches he had? Because his problem is that he still lacks maturity. He’s not so consistent as he could be. And the coach is the key: if Van Gaal is able to organize the team and the defensive line properly, then you got a good player. He needs orientation but, in a good team, his technical and physical skills will appear and you will be happy with him. Mostly, you will find him a very complete central-defender. If you don’t manage to become capable of playing with your guys as a team, then you will have problems: Rojo will start to stay off his position, he will start to go everywhere (even at the midfield) to win a tackle, and there will many times be a hole in his spot.
There’s a last thing: Rojo has played in the World Cup as a full-back, for his national squad. I hadn’t mencioned it yet because I don’t believe his offensive skills are good enough for him to succeed in a club like yours playing in that position. Your 3-5-2 system is actually a very good one for Rojo’s skills: I bet he will be playing as a central-defender, near the left, and that will allow him to play his defensive game even better, knowing there are two more central defenders in case some forward manage to drag him (something is kinda vulnerable, as I said) to another spot.
I strongly believe Marcos Rojo and Phil Jones will make a good partnership in your defense. What I don’t know is if Johnny Evans will be a good third element. Atending to his caracteristics, Rojo needs one.
PS - I apologize for the many mistakes I probably commited using my english. If it matters, I’ve done my best 
Regards,
Gonçalo