It would be nice to keep Lucas around but I don't think that will make sense numbers wise next season, due to the CL squad places...
Uefa CL squad
rules say this:
No club may have more than 25 players on List A during the season, two of whom must be goalkeepers. As a minimum, eight places are reserved exclusively for "locally trained players" and no club may have more than four "association trained players" listed on these eight places on List A.
According to several sources it would seem as if the players we're targeting in the summer are a CB, LB, CM, AM, and LW. For the sake of this outline lets say that the players signed will be those we keep getting linked to: Van Dijk, Sessegnon, Keita, Ox, and Brandt... which would leave the squad as follows (assuming Sakho, Moreno, Lucas, and Sturridge leave as expected):
Mignolet
Karius
Clyne Matip Van Dijk Milner
Trent Lovren Klavan Sessegnon
Gomez
Henderson
Can
Keita Lallana
Wijnaldum Grujic
Mané Coutinho
Ox Brandt
Firmino
Origi
Ings
In terms of Uefa squad places:
17 non-homegrown places1. Mignolet
2. Karius
3. Matip
4.
Van Dijk5. Lovren
6. Klavan
7. Can
8.
Keita9. Wijnaldum
10. Grujic/Gomez (Gomez will be eligible to be listed as club-trained when he's been at the club 3 years, from 2018 onwards)
11. Mané
12. Coutinho
13.
Brandt14. Firmino
15. Origi
16. Ings (could also be listed as FA trained)
17. Oxlade-Chamberlain (could also be listed as FA trained)
FA trained/homegrown18. Milner
19. Lallana
20. Henderson
21. Clyne
Club-trained22.
23.
24.
25.
^ no senior players are eligible to be listed as club-trained, but from 2018 onwards Gomez would fill one of these spots.
List B (unlimited # of youngsters)Trent, Woodburn, Wilson, Ojo, Ejaria, plus
Sessegnon would presumably also count as List B as he's still 17.
Clearly, if the squad is to be upgraded with the new signings it needs it'll be very tough to squeeze someone like Lucas into the squad, unless Klopp wanted to leave out Grujic and Gomez from the European squad, or simply have Lucas around for domestic games.
Incidentally, this (hypothetical) list also highlights the value of signing British players, both young ones like Gomez and Sessegnon, as well as senior ones, as that allows the squad much more flexibility in terms of numbers.