Yes, it's too early because Diego Alonso was only appointed by Sevilla last October. If they immediately decided to fire Diego Alonso this quickly, they should act quickly to replace a coach who has good qualities and that's not easy if they recruit with the hope that Sevilla will immediately increase drastically this season, this is very impossible because knowing Sevilla's performance situation which is not as strong as last season.
In my opinion Diego Alonso needs to adapt first because this coach has only been at Sevilla for 2 months and the management should not immediately take action like that because if they change coaches too often in a short time, it is not impossible that their performance will decline further.
Indeed, failing to qualify for the Champions League and Europa League competitions was a painful blow for Sevilla, but it can't be helped that they are in a quite difficult group and Diego Alonso has not yet given his best. But at least Sevilla can focus more on La Liga because their current position is very unsafe in 16th place and level on points with Càdiz who are in the relegation zone so they should at least give Diego Alonso enough time to adapt to the team.
When a coach is called up by a team, it's believed that he's got just one work to do which is to take the team to tournaments and be victorious at every level of games they find themselves. Sadly for Sevilla's boss, his position is threatened because he's failed to do his only job right. Sometimes it may not necessarily be his fault but as the manager he's got to take responsibilities for everything.
He's not really stayed so long at the club though, and he's not got the chance to prove himself but the time frame he was given has more or less elapsed. The team management wants progress and so they've got to get themselves other options maybe a positive change would happen. The club would just have to quickly find a replacement too. This time they are now is critical and it's not so healthy for a slacking club wandering aimlessly without a manager.