The four-time race winner broke his shoulder after a collision with Luca Marini in the sprint race at the Portuguese GP last month.
Having missed the previous two rounds due to the injury, Bastianini was cleared by doctors to race this weekend following a test on a Ducati superbike at Misano on Tuesday.
However, Bastianini struggled throughout Friday's practice sessions, ending the day down in 20th on the combined order, 1.3 seconds off the pace.
While pleased to be back on his bike, Bastianini admits his continued participation in the Jerez weekend is still unclear and he will assess this on Saturday morning.
"It felt like the Monday test in Misano with the Panigale – not really good," Bastianini said of his return to action on Friday.
"But I'm happy because I'm back on my Ducati and it has been a good day, with a lot of pain, but at the end I'm happy about it all.
"But my situation is not good, my shoulder is not strong and it's difficult to do many laps and also to be fast.
"Tomorrow morning, I have to see my situation and if it's like today or a little bit better, probably I can continue.
"But in the other case, we have to make the decision because to do some laps but slow in MotoGP, I'm here to bring some points. If it's not possible, it's necessary to stop."
Team-mate Francesco Bagnaia also endured a difficult Friday in Spain, as the 2022 Spanish GP poleman and race winner was only 13th on the combined timesheets and will have to face Q1 in qualifying on Saturday.
Bagnaia says his struggles are down to a lack of feeling on the front end of his Ducati.
"We were arriving from last year, when my feeling on this track was incredible, and today – not as much this morning, more this afternoon – I was struggling to find some consistency," he said.
"I'm struggling a bit with the front feeling of the bike. We are working on it. Sincerely, we have an idea that could be the solution, but let's see.
"In any case, I was expecting a bit more from me, from my bike today.
"But the worst thing is we are outside of the top 10 for the Q2. Sector 1, sector 2 are OK, but sector 3 and sector 4 are where I'm losing a lot of time."
