Martial is worth it to have playing LW, even if he underperforms occasionally. You don't have to buy the best players in the world if you have them when they're young, and I firmly believe that Martial is just that. Carrick is still a good tempo-setter, but that's it. Wouldn't be tough to find an upgrade on the pitch, would be tough to find an upgrade in the locker room.
As for the defenders, Jones has had some very solid games this season, and has only underperformed in the immediate games when he returned to the side. That being said, I wouldn't be crushed to see the back of him. Smalling was REALLY good last season, and I'd like to see him build some chemistry with Bailly, because they're both individually excellent, but have so far shown an inability to work as a pairing.
One final note is this:
The problem is that in these positions (CB, CDM, WF) world-class players are not available in the market
This is a horrible mindset. If you need to buy world-class players to accomplish anything, your coaching sucks balls, and United's doesn't. One here and there doesn't hurt, but when you see a spot of need, you don't have to spend 50 mil plus to correct it. That's why City's a joke, and has still somehow spent more money than United in the last few years. We should be making our own players better, and we usually can. More than a few clubs have failed because they couldn't recognize their own talent.
Which players do we need next season (Let's say we qualify for CL .. I believe we will be there)
I'm always thinking about which players we need to fight against the best teams out there next season
We need 3 or 4 players ..
LW... Martial is not ready yet ... and we need a finished product here
CDM... Carrick is gone and we need a world-class player in this position, one that will give us another dimension
CB.. Jones and Smalling are not good enough ... so we need a world-class to be the leader and to be alongside Bailly
Striker.. Antoine Griezmann .. these are the types of players we need to win titles
The problem is that in these positions (CB, CDM, WF) world-class players are not available in the market