New Position

With only one left-back in my squad and no money to spend, I would like to train a current player in a 'new position' so that they could act as a deputy to my left-back.

I have added this on the player's training page and he is currently unconvincing in this position. Over time, with him training to become a left-back, will this improve? If so, how long will it take before he is comfortable at left-back?

I think he'll be acopmplished in that position in about half a season. The more you play him there the more faster he will be acomplished there.

It takes about half a year to get a player from accomplished to natural, in my experience. This is with good training facilities, playing him at that position a lot (usually the players I retrain play about 50% of the time, all at their new position). I'd expect it to probably take the same amount of time or slightly less to go from unconvincing to whatever the next level is (accomplished?)

I think it also depends on the player's age and his versatility how fast he will learn the new position.

Unconvincing > Competent > Accomplished.

You were lucky that you managed to get a player for about half a year from accomplished to natural. It ussually takes even about 2 seasons or so. And that if you play him all the time in his new position.

And also, yes, it does depends on your training facilities, but the attribute you should consider the most IMO is his Versatility. The great the attribute, the faster he'll reach desired level (natural).

And, the youger he is the greater chances are that he'll become accomplished.

Gaining a new position depends on some (hidden) player stats like adaptability and versatility and if you keep playing him in the new position, the process will be faster. I suspect that age will, as with other training, have some influence as well.

On my current save I desperately needed some central defenders a while ago, because I had three long term injuries and two foreigners not eligible (were playing in my youth team). One of the two DL and DR I used, both were "Awkward" at DC previously, took 7 months to get to "Accomplished" while playing DC in all league games. The other one never got beyond "Unconvincing" but also only played a few games on the DC position.

I read somewhere that it can take 2 years by using the training option alone, though I've never tried that.