It goes beyond squad depth, I got promoted with Pompey using what, 16, 17 players?
Your tactics fail. You're playing VdV as a defensive CM - how the **** is that supposed to work, mate? You haven't done any tweaking to the players as far as I can see since in both your tactics several players are set to automatic, which I don't recommend, plus your formation seems to be symmetric, which, even if it's not a problem by itself, often means you're probably not getting the best out of your players.
Now, I'm no pro at this, so you should not take my advice as gospel, a divine revelation or **** like that. There are several users that can probably be a lot more helpful than me regarding tactics (iNickStuff, GodCubed and TheBetterHalf come to mind), but I'll try to help you out. I repeat, once again, I'm not too good at this. Be warned.
Your keeper's decent so you don't have much to change there. Your defence, OTOH, is **** and you're probably leaking goals. I've never been good at defensive football, to be honest I've Hollowayed my way through every single league/cup I've played, but I do try to keep it as solid as I get.
McManus and Wheater can probably be a good central pair, but Zewlakow is what, 35, 36? He's probably got 6-7 pace, that's beyond suicidal for a defender. Ditch him. Mendy isn't my cup of tea and he's probably getting on a bit too, but you can keep him for now, as a FB on support duty - he can probably play as wingback but since he's got **** marking and probably lost some pace if may be better if he didn't venture too high up the pitch. You need a LB urgently, though. Taking into account the fact that you have Vaclav Kadlec, who makes for a good inside forward (If you're not gonna play him on his preferred ST position) you should get your hands on a pacey left wingback with good dribbling and crossing, set on attack duty. I think you've cheated horribly, to be fair, forgive me if my assessment is wrong, but if it's right you might as well replace the words "pacey left wingback with good dribbling and crossing" with "Leighton Baines". There are better ones but they're not British.
Now, let's take a look at your midfield. I favor wide formations and if possible with 3 players through the center as opposed to the midfield pairing of a creator and a destroyer found in a 4-4-2. Considering your current players I'd try to build a 4-2-3-1 formation. Let's start from the left, since that's where we left (Redundancy ftw) before. Kadlec can probably do a good job as an Inside Forward, cutting inside. Rosicky can probably do that, too, but he is old. Probably lost any pace he had left. In central midfield you should probably place Thomson as a Central Midfielder or a Ball Winning midfielder with Defend duty. I think he's a good B2B too, but IMO having a good defensive player in the middle of the park is essential and I always have one. You will need a good passer next to him, which you lack atm. Either a good central midfielder with some creativity (CM Support) or a creative player with some defensive skill (DLP Support) should fill that void. A little higher up the pitch you will play VDV as your playmaker or even as a trequartista. Rosicky can probably do that, too, as you don't need too much pace for that. On the right you can probably use a typical pacey winger hugging the line, which you will need to buy.
Finally, you have plenty of choices for the single striker position, I think they're all good so I'll let you take your pick.
And that's as far as we'll get today. Hopefully it'll help you, and if it doesn't, I'm sorry, you may try contacting one of the three names I wrote at the top, preferably in the Tactics section.