I would definitely keep him here this season and have been saying as such all summer. Also could see him drop into that Lallana role with age too.
I was looking around today for a comparison for Kent and ended up on Asensio. If we can get a similar season as that, we´d be laughing. Asensio had about 10 starts and another 10+ cameos (1300 minutes total in league and champions league). I wouldn't expect the same output but just someone who could take some minutes off the stars and pick up 10-15 games worth of pitch time.
In terms of output, Asensio got 6 goals and 3 assists (0,61 G+A/90) which is well on the path for an elite versatile forward player aged just 21. We can´t expect that of Kent yet clearly, so what of other young wingers in the EPL? Rashford, by way of comparison was 0,31 G+A/90 in the league (0,36 if you include Europa League). Some of those minutes were playing CF too which usually results in better output.
Zaha´s last 3 seasons have been 0,27 G+A/90 (age 21), 0,11 (age 22) and 0,47* (age 23).
His numbers are a good example of how variance can make players appear better or worse than they actually are. Therefore judging him on last season comes with the red flag that his expected numbers were much lower which means he may just have been a result of variance. His expected numbers based on the quality of chances he created and shots taken were actually 0,31 G+A/90 (age 21), 0,26 G+A/90 (age 22) and 0,30 (age 23) G+A/90.
So what should we expect of Ryan Kent? If he does hang around next season, expecting a goal or assist every 4 games or better would be a great start. Therefore if he gets about 1,000 mins of football you are asking for maybe 1 or 2 goals and the same number of assists. It doesn´t sound a lot but that would put him on the path to being a solid Premier League winger and around about what we would realistically get from someone like Zaha if he signed and only played the same amount of time. Likewise, Demarai Gray on 0,25 expected goals + assists per 90 and Albrighton on 0,22 show you what an average wingers production would look like in the Premier League.