Another Celtic fan here, sad to see him go but I like Southampton so hope he does well for you, I have no doubt you will be in the same position as us in 24-36 month, those FM stats are a joke, he's got all the attributes to play at the very top of world football.
His only weakness is he can be a bit lackadaisical, but I assume that was due to things being very easy for him in some games, when he's concentrating 100% he looks as classy a defender as i've ever seen.
Not sure if it's Koeman's style but if you are needing a goal late in the game Virgil is a very good targetman, I'm certain he could do the job as a striker in the right team.