Squad Rotation tips?

I'm getting a lot of injuries this season, since I have only 5 defenders. Heavy bidding wars of TM even now, looking forward to negotiations next season. I'm still alive in all 3 competitions, that's why my players get exhausted. I have only 1 GK (the other retired, couldn't buy any since then). What I've learnt is that we shouldn't go to bidding wars.

I usually play 4-2(MC)-2W-2, and sometimes 5-2-2-1 in cup home matches, not to be concerned of. How should I build up my squad for the next season?

PS how can I post pictures from Android 4.4.4 using Opera? I may upload my squad.

There are many alternatives to build a team. You may choose yourself a style you like.

The simplest approach is keeping a full squad with players for two 4-4-2 formations. Then even if you face quite severe injury problem, your team can still use formations which are not too weird.

I tend to keep full squad. I believe this may be the actual solution for many managers who complained about injuries. In the past, I heard that team will suffer financial crisis in high levels that they cannot keep too many high wage players. But I still don't feel the problem after Nordeus increased the competition prize and my teams' cash balance are kept increasing.

If you are afraid of bidding wars, you may buy less attractive players (those with qualities end with 0,1,5 or 6). If you are lucky, you can get unexpected fast trainers from these players.

For this season, you can also temporarily use the old free agents. These players are 3 star but many old players are really consistent and reliable.

As cookizzz said, there are many alternatives to build team, so I'll present you some of them which in my opinion are the best choices.
To be able to rotate team, you should buy as much as you buy up to 21 (included 21). Leave free space for 1 player - academy 0T player of free agent. If your team isn't financially strong, keep eye on cheap high quallity players or players which are on auctions for low % of their value.
If it is possible, buy 2 or 3 position players. 2 position defenders and DC-DMC are most often type, they could be very useful.

Single formation based squad
If you don't mind playing whole season with single formation, then this strategy is made for you.
For this strategy 1st logical choice would be 4-3N-3. Transfer market offers many MC and ST so it is easy to find and buy such players. You can also try this strat with 413N2 or 431N2 if you don't want 6 strikers in your team (3 in game, 3 on bench). There you'll need 2DMC or 2AMC instead.
Such squad should have:
2x GK
7-8x defenders - usually 4DC + 2DL + 2DR
5-6x MC
5-6x ST or 3-4ST + 2AMC/1-2DMC

Various formations playable
This strategy is one that I use and I won't change that strat. Team consists of players for all positions in order to be able to play any formation you want. This strategy needs multiple position players, as much as possible. Main flaw is that finding and buying players for some positions could be complicated. Transfer market usually has poor offer of some positons like AML or AMR and there are many bidders waiting for such players.
My teams have same core of formation, that is 2MC+DMC (or sometimes 3MC). That means I use formations like 451V, 352V, 4141, 3N142 etc. So I always buy at least 6 central mildfielders (4MC + 2DMC). Those positions are constant of mine formations, the rest I adapt to opponent, choose to play with 3-4 defenders, 1-2 ST, 2 or 4 wings etc.
So my squad usually looks like:
2x GK
7-8x defenders - usually 4DC + 2DL + 2DR, mostly double position players.
4x wing mildfielders (usually ML+MR+AML+AMR)
4-5x MC or 4MC+AMC (if AML/R can't play AMC)
3-4x ST

Booster managment
If you don't farm or buy boosters, then you need to spend them wisely.
When you set up your squad, you'll try to keep in 1st team most useful players (ones with good both quallity and form). If your schedule gives large time difference between 2 matches in day, save your green boosters. Your team could get 15-20 condition points when one match is in the morning and 2nd in the evening. If you have 2 matches in 3 hours, then spend 1-2 greens on important players which played 1st and put them on bench or even in 1st squad ih both matches are hard or if your b-squad players are out of form. For 2nd match I usually try to have ~90%+ condition of 1st squad and ~80%+ of substitutes. Don't spend greens on players which are going to be reserves for 2nd match. Also you don't need to spend greens on GK which is on bench because he likely isn't going to play matches.
If you have some young fast trainers, then they should be primary players to get boosters because it will pay off in their quick growth.

t11_fan... I generally stick to one formation which may change every season (last season I preferred 4-5-1V, this season it's 4-2-2-2), formation may change according to condition (If I face a team that concedes a lot of goals I will play 4-3-3, for example). Thank you for the green saving tips (BTW I have only one GK... still bad wars).
cookizzz... THANK YOU SO MUCH FOR THE TM TRICK... got 2 defenders now I can finally rotate them.